It would depend on how the older boy was--was he deliberately trying to be mean or was he oblivious to your son unless your son tried to take "his" (other boy's) toy. The reason I say this is that what happened to your son is a common dynamic between my 7 year old autistic son (who is probably on a 2 year old level socially and the size of a 4-5 year old) and my 19 month old daughter. He'll be doing something with a few toys and if you take one toy out of the bunch, he has to get it back or whatever he's doing won't "work" in his mind. 90 percent of the time, unless there are screams and blood, I let them work it out themselves (usually one or the other bores of it quickly and finds something else--when I interfere, then they draw it out because mama's involved, and it is now a game).
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Of course, with a child you don't know, you wouldn't know if there was a handicap involved. Yes, my son "looks" completely normal--as long as you don't try to involve him in conversation (he is functionally nonverbal--he can say some words, but cannot have a conversation--you can explain about sharing all you want and taking turns, but you may as well be speaking Martian. And if you just confiscated the toys, even after explaining--all you are going to get is a meltdown, think tantrum on steroids that takes about an hour or more to come down from, and a pissed-off mama because our day is ruined) and he isn't stimming. I might have seemed oblivious because my main concern would be paying attention to the person we are speaking with and ensuring our son does not run out the door. As long as he isn't running out the door or destroying stuff, we are having a pretty good day.
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Because of my personal experiences, I'd get the other child's parents involved. Nicely explain the issue and ask their assistance. If you are both at the apartment rental office, you may soon be neighbors. Wouldn't you rather know that they are "awful" people that you wouldn't want to live near--or you could find out they are actually pretty nice. I'd have come over and either helped by doing our "share ritual"--play for a minute, then say "one. two. three. AFWife'sson's (or my) turn." and they would switch. If that didn't work, we'd take him to the car.