My son has PDD- NOS, and when he started the same type of eye blinking thing I was really worried that he had developed a facial tic. It really scared me. At first I thought it could be him just copying the beginning sequence of the move Hairspray (his absolute favorite), but after it persisted for a few weeks I decided it had to be something more. After doing some research I found that when little boys, who are very sensitive to metals, don't get enough magnesium sometimes they develop facial tics. Once I started introducing more magnesium into his diet the tics stopped completely. Then, a few months later I stopped regulating his magnesium intake and the tics started again! Within a week of getting "back on the wagon" his eye blinking thing was gone! I don't know if this is what is wrong with your son, but it is worth a try.
Go to this website: http://www.ctds.info/tics.html
It will give you a lot of helpful information.
Could he be copying I have seen this kind of behavior before and sometimes a sibling is doing it to teach the little one to do the same ,they think it is a game, grown ups even have this little idea that making a child do something silly is amusing , check out if anyone is playing Games with your child.also ask Teacher at school as other children there may be doing it and he is copying, Other thought is has your child been taking any meds as there is sometimes a connection there,PLay it down if he finds it gets him more attention, he will do it get that. Good Luck