I hate wetsuits. I have a vest I wear sometimes when the water gets down to what you are describing. Especially if you have not done any wetsuit swimming, you need a stick of Glide to lubricate the friction points, which for me is upper shoulders and around the arms. Your son will also be well-served to wear in the pool a couple times because the buoyancy will change his swimming some. They actually promote body alignment better, but it's a different feeling. I was swimming outside in 65 degree water last September, and it was challenging. He might also try double capping to keep his head warm, but be wary of wearing a silicone cap under a latex cap. He may lose the latex cap. Again, try it in the pool first. My daughter did the Smith Mountain Lake 5k in October and lost both of her caps.