It would be easier to explain it to the child’s parent & then hopefully she or he could teach their child about your son & his specific ways of play. Most toddlers aren’t receptive to adult strangers, for example if you are at a local park.

A lot of kids have a sibling or relative with autism so they may have associations already. Toddlers are a very inquisitive group so they may guide the discussion in some ways and the answer would depend on your child's personality.