I cannot get a 2 year old to PLAY

[5 Little Monkeys]

I have one whose been here since last June (he's 20 months now) and he is fairly quiet here. He tends to play alone or on the side lines. He sits in the library chairs and reads a lot. Although, he is slowly starting to come out of his shell. Outside is a different story though...he's off and running and plays a lot with the kids. I've commented to mom about it and she said at home he is always busy and playing loudly.

I think maybe it's just a numbers thing...he's an only child so I wonder if maybe sometimes the other kids overwhelm him? Outside he has more room to run and play and he really loves the outdoors so it's more in his comfort zone.