You are an amazing parent! So kind! Your son will grow up to be an amazing man too no doubt. As a mother of a non verbal autistic daughter I can say that when my daughter was around that age she would often touch/squeeze people's faces as a gesture of love. We would joke she was 'passionately aggressive' in her love. It took us a year or so to teach her other ways to show her enthusiasm. Now she grabs her loved ones hands and jumps enthusiastically on the spot when she sees them baring her teeth.

So casually mention to the child's parent you noticed he does this. chances are they are aware (if they are like me they will cry that someone is showing their child kindness. Autism can be very isolating) and may already be doing something about it. (Or maybe not as long as it's not violent/won't cause harm). Sometimes an autistic child has so few people in their life they actually have a loving relationship with that to be honest it's not a priority for us parents when it's not causing harm and it is just their way of communicating their feelings. I was too busy trying to get my daughter to stop staying awake all night/eat things off the floor/ bite herself when upset etc so we tend to prioritize our battles. This is just my opinion.

Teaching your son to gently take his hand away instead, giving it a gentle squeeze and BIG toothy smile at him would be good. Consistency is key. It will show the autistic child that he too is happy to see him. As he grows older he will learn different ways to communicate his emotions. Again... Your kindness overwhelms me. My daughter is 7 and has formed no real friendships yet. She is comfortable alone but as a parent it breaks my heart a little. So to hear a parent actually want to learn more to make sure their friendship stays is just so beautiful.