He needs to get coaching. For someone in physical shape, the biggest obstacle to overcome is the stroke technique. Very few people outside of the competive swimming world actually swim efficiently and that will make your sons effort a lot easier. I would definetly recommend looking for a local masters program-- Tampa Bay Aquatics, Forest Hills Aquatic Masters and Distance Matters are all local clubs that could help you out...a University Swim Coach would probably help also, or hook your son up with one of his swimmers for assistance.
And for what it is worth, I would higly recommend your son go PJ/CC--great training, great job and--well--no gate guard duty.
Good Luck