During plebe (fourth class, freshman) year, your son will rate (earn) leave over Thanksgiving, winter break, and spring break unless he is on restriction, has other duties, or travels with a sports team or an extracurricular activity. He may also get leave between the end of finals week (after LMO or last military obligation) and Sea Trials, which is usually the week before Commissioning Week; and again between Sea Trials and Commissioning Week (but my understanding of this is fuzzier, so current or recent mids should fill in).
Summers are divided into four blocks, Block 0 through Block 3, each approximately 4 weeks long, with Block 0 being much shorter.
Here's a link to Summer 2015 dates - obviously these will be different next summer, when he's a rising youngster. During these blocks are when summer training (2 blocks) and leave (1 block) are scheduled, as well as mandatory or optional summer school, co-ops or internships, NOLS trips, that sort of thing. Generally training in Block 0 is reserved for athletes in intercollegiate sports and a very few select others. Sometime in his spring semester, he will express preferences for training activities (to the extent he can) and dates. He will find out what he got at some later time. Mids are supposed to get three weeks' leave in the summer. However, in ALL things about USNA leave and more broadly, leave while in the Fleet, it is super important to remember to be flexible. Plans can change by a day or two or a week because he can't get on a transport, or there's a storm, or myriad other reasons. I've heard of mids having, rarely, 5 or 6 weeks' leave one summer, or a few days instead of three weeks. It's rare but it happens.