Yet another Marine writing in to agree that it's best to leave the marksmanship training up to the PMIs. As long as he's not actually afraid of firearms, lack or firearms familiarity should not be a problem, and might even be an advantage.
However, once he gets out of Recruit Training and MCT, it's very possible that he'll have a renewed interest in rifles and rifle marksmanship. Unfortunately (as a young MV found out a few years back), the military lifestyle is not conducive to firearms practice on your personal time. Mainly due to the difficulty of owning a rifle on base. However, should he end up on a base with a recreational rifle range, or civillian ranges close by, an AR-15 might make a great gift someday. Unless he's stationed at Pendleton or 29 Palms, in which case... um... any CA-approved rifle would be great.
If he's living in the barracks, he'll have to keep the rifle at the armory or at a buddy's house. Some armories will only let you check out weapons at pre-set times, some states frown on keeping weapons at others' houses. So the individual situation will vary greatly.
The best things he can do prior to going are to work on running, crunches, and pull-ups. Eat right, get all the nutrients for strong bones: stress fractures are a sure ticket to several joyous months in MRP (medical reconditioning platoon) before resuming training.
On the non-physical side, ask the recruiter if he can get the book of "knowledge": the assortment of Marine historical facts, military law, first aid principles, etc. The major academic requirement of Recruit Training is to be able to pass an exam on the above subjects. It was called the "Prac Test" a few years ago. If he's at all decently bright, he won't have any trouble passing the test. But it never hurts to study the book beforehand; one less thing to worry about.
When all is said and done, all a man needs to graduate recruit training is decent physical condition, average intelligence, a receptive mind, and the willpower to keep going.