It sounds like he is stimming. He may fall on the spectrum or have some sort of sensory processing disorder. If it doesn't effect his day to day life then I wouldn't worry about it. He may look silly but if he isn't hurting anyone then just ignore it.
When he was younger he would do it while watching like nascar baseball game like when someone scored and etc and he would do it right in front of us like he didnt care lol he would even do it outside and he lived right by a highway but now he makes sure noone is around when he does it ive read topics of kids doin it but hes in his mid 20s so got me curious and ty for the reply =D
My Dad has an arm flapping tick too but growing up hes found ways to "hide" it. I never knew about it until he told me when seeing my 5yr old showing the same thing but when he was younger. No one knows what causes it but its not from excitement like what you describe. Anyway, Im thinking that the reason you havent seen it lately isnt because hes not doing it around others but because hes learned how to disguise it.