All three rounds are going to be equally effective as a generic deer rifle. From a purely practical point of view, 7-08 makes perfect sense as you're already setup to load the round and are familiar with it.
But, there's something to be said for getting a new chambering and starting from scratch working up loads, especially going through the process with your son so that he can have that experience. So I'd say the 6.5 creedmoor deserves consideration.
