He should have plenty of room to move Dee, they are really very generously sized. It may just make it a little tricker for him to crawl properly because his legs are inside it, however they really do have a lot of room to move.
I dont think its too old to introduce one, as Aidyn used one up until this summer (about 18 months of age) and is going to continue using his when the weather gets colder again.