I have a 3 yr old dauther sho has in the past years also flapped her arms and ssssoes almonst the same as your son. She does not talk as yet. She sees a therapist for her speech but to no avail has her language developed as yet. Needless to say I am worried. I have read from Thomas Sowell that he had a son that did not talk until her was 4 and that some shildren have childhood wuirks like handflapping especially during solo play, like my daughter. He also states that this is something that most of these children grow out of. This is s sort of self stilmulation that these children do. It sends messages to the brain and sort of comforts them. After reading this I did not worry as much. My dauther does not seem to do it as much. She also banged her head alot but doesn't dddo that as well anymore. She is very social and loves to be with other cildren unlike children who have autism. So I beliwve is your son is social that he will grow out of these ciildhood quirks.