Usually, this issue comes down to coaches and scholarships. Both sports train year round, so coach giving scholarship won't allow athlete to miss a season for the other sport. This forces kids to choose.
Some who had choice were Gwen Jorgenson, Morgan Pearson, and Renee Tomlin. They all ran in college, but ended up moving to triathlon post college. Cycling is the one sport of the 3 where the aerobic motor tends to transfer to success. Takes about a year, but doable.
I'm surprised your son does no weights. My kids go to a middle of the pack high school, but both running and swimming programs have weight training and flexibility programs.