The Navy is different from the Marine Corps, but there are enough similarities that I feel comfortable answering this question. Mostly it comes down to numbers. Where are the numbers needed and when are they needed?
1) The recruiter will need more recruits during those months that other possible candidates will be in school and unable to attend boot camp.
2) During the summer months more people go to boot camp and it's possible that it's just full until the time you were told.
3) The recruiter already made his quota and needs to save your son for a month when recruiting is particularly hard. Recruiters get credit for recruits when they finish boot camp, not when they leave for boot camp.
These are just a few possibilities and I hope this helps. I would recommend that since your son has some extra time before going to boot camp, that he try to recruit a few people as well. It's called recruiter assistance and he can earn credit toward a promotion before he even goes to bootcamp. Just make sure the recruiter gives him his due credits!