RE: Caliber for son's first deer rifle?
Caliber?....Anything bigger than .24 is probably legal, and anything will kill a deer.
More important than caliber I think is to find a rifle that he likes and is comforable with. Take him to a store with a large selection and let him pick up and hold anything you can afford. Eventually he will put one to his shoulder and say, "This feels great!"
When that happens, pick a caliber.
He may pick up a Marlin lever action and say, "I love this gun.", then get a 30-30.
He may pick up a Remington bolt action....Get a .243/7-08/30-06....
Caliber Shmailiber...get a GUN he will love.