The better he wants to be, the earlier and more frequently he should be running. A few days a week for the first couple weeks building up to 6-7 days a week and increasing how many miles he runs. Dictate the increases by how he is feeling, but a rule of thumb is 10% increase in miles run a week. This is the time to build a solid base of aerobic "easy" running. Hard workouts aren't necessary.
If the team is any good they will have unofficial and/or official team practices in the summer. This is a good time to meet the team and get ready for the season.
Find out as much as he can about his future coach and teammates. Buy him a book or have him go to the library and read about training and race strategies. Be a member of a local RRCA running club or get into a summer Jr. Olympics track program so he can listen and learn from several coaches.
Run over the summer because it is fun, not because he is obsessing and stressing about impressing.