It is going to be one of three things: due to the device on its own, due to the electrical signal from the device, due to the wristband or due to a placebo. If you really want to isolate this effect then approach it like a scientist would and spend a little time and really isolate the effects. Here is how. (I've written this quickly so hopefully I've covered everything).
As a scientist, I would first test if he gets the same effects if the fitbit is put on the other wrist and if he gets the effects if he puts it on his ankles (I'd call that test 0). I'd call this test 0 because it may solve your situation but whether it does or doesn't the interesting reasons for what is causing it still need exploring. These are done by the following tests.
Test 1, run the fitbit down so that it is out of charge then put it back on your dad (original wrist) and don't tell him that it is chargeless and see if he experiences the same effects. Tell him not to touch it or look at the face (maybe cover the screen if you can ideally) or it may mess up what you are doing (so he doesn't know it's chargeless). If he does experience the same effects then it is likely in his mind, believing that it does something when it doesn't (placebo). But if he doesn't experience any negative feelings then that's very, very interesting. The body is filled with electric and magnetic fields that are natural but i'd imagine (I don't know what signal strength used by the fitbit) that the fitbit wouldn't have much effect, if any, but you never know.
So if he does experience an effect with the chargeless fitbit on then I would take it a few steps further. Perhaps he is affected by the band. Obviously with his permission but without him seeing it directly I'd do the following as an experiment. (Test 2) I'd remove the straps from the fitbit head and I'd put the straps back on without the fitbit head (so only the straps ad no fitbit device) and join them so they are touching all the way around his wrist. Perhaps use some elastic bands to hold them in place. Distract him when doing it and tell him you're testing something and not to look. I'd put a bandage around it so he can't look at it and tell him not to touch it (tell him not to tamper with the bandage or it will mess up what you are testing so he doesn't remove it or tamper with it). Now see if he still gets the effects! (If you want to be really thorough try just the bandage and then try just the elastic bands on their own as well and try the fitbit bands but not joined around the wrist but I don't think you'll want to do that much and it's probably not necessary).
If he does experience the negative feelings then try test 3 which is try the uncharged fitbit head only (no wristband) on the arm taped on but don't tape around the wrist (very important), rather use a small amount of tape parallel to the arm (choose tape that won't mess up the fitbit device). Then (test 4) try the same as test 3 but this time with the fitbit head charged up.
Results
This is what I would draw from these tests:
Test 1. This tests if the uncharged fitbit has the same effect. If he experiences no effects then it is possible that the (likely) small current used is affecting him! It would surprise me as we often have things in our environment that can produce small changes but, hey, that is the conclusion I'd assume at that point. I would then do Test 5, try the fitbit charged and uncharged on his other wrist and on his ankles. It may be a matter of finding a place that doesn't affect his internal natural fields. If he gets effects from all these places then I'd be very interested to know that result since that would be very surprising and interesting!
Test 2. This tests if the wristband is having the effect. If he feels an effect from this test then it is the strap that is affecting him. Since the wrist bands are likely insulators (judging from looking/feeling them) then it may be a response of his skin to the material. In that case, maybe try putting something under the band part (but not the device) so his skin doesn't feel the plastic. The other possibility is that the insulator band alters the internal magnetic fields but I'd be again very surprised. From test 2, if he doesn't feel any bad effect then it is likely due to the fitbit device and not the wrist band. Tests 3 and 4 will help isolate that.
Test 3. This tests if the fitbit device as an uncharged device is having an effect. If he experiences a bad effect with just the fitbit device on his wrist and no wristband then it's likely due to the fitbit (or the tape which you could test on its own as well) but the fitbit is uncharged so I would take another step of taping on his arm a small coin, try a 1 cent or penny then try a dime or a quarter or 25 pence piece. He may be reacting to a conductor being on his arm because the fitbit has no charge it is essentially a slab of conductor and can produce a minor image charge on its surface in response to fields nearby under the skin but I wouldn't expect this response to be of any significance... loads of people wear necklaces or rings could in principle do the same but I'd expect this is a very very minor effect and negligible. If you want to be ultra rigorous then I'd try with just the tape you used and no fitbit device and no wristband. If he experiences again then he is reacting to anything there so it's likely to be a skin reaction.
Test 4. This tests if the charged fitbit device is having an effect. If he was fine with the fitbit on its own taped on the arm (result of test 3) then this test 4 will show if charging it up alters anything. If he is fine with test 4 (no bad feelings) then he is fine with the device and it's likely the wristband. If he was fine with test 3 but not test 4 then it is potentially the electrical effect from the fitbit.
If you suspect it's a placebo then try the following. Tell him that you want to test the bandage on its own and do that. Then tell him you are going to put the fitbit under the bandage but don't. You could perhaps make the bandage thicker or put a bit of cardboard inside the bandage to make him think it's in there. Tell him that you're trying the fitbit without the tightness of the band. if he experiences an effect then it is really likely placebo. The last thing I would do is tell him in the bandage there is a small weight but no fitbit. And do just that. Put a small weight in there and see if for some reason he is sensitive to weight on his arm. I can't imagine why that would make him feel not good but if he was aware it is just a weight and not a fitbit and had an effect then it's not a placebo it's a real response.
Okay, I have no more time to think up tests for now. I'd really really be interested to learn if anyone actually takes the time and effort to test this like a scientist would. I'd be happy to correspond with anyone if they do undertake these tests. We could really find out the origin of these negative feelings if people do this rigorously.
