I don't mean this to sound stupid, but are you sure he's not just doing it because it looks cool? Different lights operate at different frequencies and can cause different effects if you wave your hand in front of them. Try it in front of an old CRT computer monitor; it has a strobe light-like effect. LCD monitors don't. It's a similar effect to staring at a ceiling fan (on a low speed) and blinking really fast; it has a strobe light effect.
It could be a tic, but it could also just be that he's discovered a property of different frequencies of light emission and he's messing around with them.
Of course, natural light wouldn't have this effect, so it could be a tic since you said he does it for natural light, too. And the fact that the neurologist diagnosed it as a tic means it probably is.
How often does he do it? Every time he encounters a new light source? Only when he's board? How old is he?
He just turned nine and has been doing this for 2.5 years. It started while at the pool and bright sunlight has the greatest affect. He tics a lot, everyday. At times it seems uncontrollable and is exacerbated by stress and excitement. However, there are many times he seems to get some other sort of need met by doing it. I say this because it's slower - the head goes back - than you know the hand will wave. When it's slower and seemingly more deliberate, he waves his hand longer. When it seems uncontrollable, the movement is quicker and shorter. Does that make sense? As I don't have any tics and actually, have never talked to anyone else that has (which you may find odd) I don't know how it feels and since he's young, it's hard to discuss. I keep going back to stress. Could it be some sort of stress relief?? I was just got a bottle of Melissa Supreme by Gaia's Children. It's supposed to help with stress and also has scullcap in it. I'm very, very new to all of this - especially the herbal remedies - and I don't feel like I know what I'm doing other than to keep trying different things. I would like a professional I could go to. My son doesn't fit "neatly" into any categories and I the professionals I've spoken with don't seem to know what to do. I will not give him medication. Any suggestions would be helpful.