Things worked out beautifully! We only had one basic rule: that if he started to fill overwhelmed that he went to his room or came to talk to me. I also asked him if he would feel upset to share any toys with the other boy and he said he wouldn't. When they arrived, my son and the other boy both took off to the play area upstairs and played very nicely! I could not believe it! Although my radar was up, I tried my best to give him a sense of control. The boy was his guest and I was not going to interfere.
He was also great with the baby. Only a little disturbed when the baby started to cry.
Toward the end of the visit, he was tired. He decided it was time to watch a movie. The tape was not working so well and he almost had a melt down. I could just see it starting to build up. But we fixed the tape and he calmed down again. The credit was all his for bringing himself back. He really wanted to be a good host. It was very cute to see that.
Thank you all for the suggestions! I think next time I will keep it a lot shorter because this was a bit too long. They were here for about 3 hours or so. That's a lot for a 5 year old to handle. And thanks for the link for the social story. I will certainly check that out/