What to tell my three year old son about child with autism??

I've been going to this parent group thing where your kids play in a big room of toys. My three year old boy (Liam) gets along well with all the other kids and leave them alone for the most part. Except one child. He's a four year old boy with autism and extremely protective over the toys that he plays with. It doesn't help that they both absolutely love cars and toy trucks.When my son wants to play with some toys that the boy is playing with he'll yell and take it away from Liam. I will show Liam other toys he can play with and when the little boy is finished with the toy he can play with it. Even if the boy is done with the toy he'll run over and still take it from Liam. To be frank he's quite mean to my son.I'm not really sure what to do at this point. It's been ongoing for a while now, I've talked to his parents and they're really trying to stop him from being so possessive and whatnot. How can I explain to my son what's going on that a three year old will understand? via /r/Parenting https://ift.tt/36h79n6