Thanks ladies, so it's unanimous then- paranoid mom!
The main reason I was concerned is that I was worried that it was a lack of oxygen that was making him so to sleep so easily. I think he is sitting up with the blanket over his head, then lies down or falls asleep and then lies down so it's really quite hard for me to get off sometimes as a bunch of it is underneath him. When I ask he says it makes him warm and he falls asleep easily that way.
Okay, I guess I should stop panicking!