
So long as he's lively he's probably ok. He should be nice and plump, you don't want them going like gangly teenagers for a while yet, but don't just stuff him in the hopes of him catching up (I'm sure you wouldn't!), check with the vet like you say. I've seen 2 year old Cavaliers that are as tiny as 6 month puppies, and as long as he is healthy in himself you shouldn't worry about having him perfectly in the middle of the breed standard or anything.
ps still waiting for photos when you get them off your camera! :-)