I’d think it would be very, very hard to swim outside of the SPS structure. One, your child would be required to participate in SPS athletics anyway. Two, the time to get off campus and participate would be challenging with the busy schedule of classes, athletics, ECs, study time, etc.
Have you and your son considered Deerfield? Not as big as Exeter and Andover although bigger than SPS.
Great idea, @doschicos! Deerfield is an awesome suggestion. Perhaps @GnarWhail can check out nepsac schools with swimming and cross reference against size and the schools that her son prefers. (http://www.nepsac.org).
I honestly think it would be very challenging to swim USA in conjunction with BS unless she moves to a neighboring town and has her son attend as a day student or if she ferries him to/from USA practices. I have seen that be successful when the parents literally relocated to support their child’s swimming. However, in those cases, the children were exceptionally gifted swimmers whose times were competitive with being recruited at the Division 1 level in college.
If we ever relocated for swimming, it would not be to the borders of a NE prep school campus! Any relocation would need to be a sort of place with year-round outdoor swimming and pleasant, mild temperatures when the sun is coming up during morning practice in the winter.