My daughter has autism . she hates touch and physical contact . I guess every child is different. Also i worked with adults with severe autism years ago and they did like physical contact , sometimes it would be hitting though rather than a hug . they struggled with vocal communication so I guess they used physical contact to cimmubicate in other ways .

Maybe your small frame made you less intimidating . maybe something about your facial expressions and body language showed empathy and it sounds like the arguing distressed him. My Austistic daughter hates loud noise and distractions .

Try not to overthink it and if he does it again and it makes you unconfortable try and ask his mum why he does it . it just occurred to me that maybe you look just like somebody he knows and he may have mistook you for that person.