Sounds like he is delayed in play due to his circumstances. He is an only child at home and has been around younger children in his day homes. Sounds like you are doing a good job at teaching him how to play though!

I would keep doing what you are doing and encourage him to play with the others. If there is a specific toy that they are fighting over, I will either put it away or bring out a timer and each child gets 5-10 minutes to play with it before having to pass it on. If they still fight over it though, it gets put away until the next day. I also have to say multiple times a day "find a toy that no one else has right now" to my 4 yr old dcg. She loves to take their toys if she sees they are having fun with it. It's annoying so I know how you feel!! lol

Could he be bored? Does he play with age appropriate toys?

Keep doing what you are doing and do lots of shadowing. He will get it eventually!!