My son who is a adult with autism, i notice his left upper are seems to be swollen or larger than his right upper arm, and a small bump ?

A Stitch In Time...: If he exhibits a lot of pain in that area or if he is reluctant to use that arm or if the skin feels warmer than the other arm or the swelling increases then have him examined by a doctor. Sounds like he bumped or otherwise injured that area or an infection is brewing so if you have any doubts about it, get him checked sooner than later.