Go where they go. And that's not the doctor's office. Try parks, basketball courts, barbershops, community centers.
Make waiting rooms more male-friendly. For the rare times they are in a doctor's office, add magazines that guys read (Sports Illustrated, ESPN Magazine, or anything about music, computers or gaming ). Put up sports posters. Hire male support staff. Young men are far likelier to open up about their sexuality to other males.
Give boys leadership positions. Teen councils sponsored by Planned Parenthood, for example, are hungry for male members. And guys are much likelier to make inroads with their male peers.