So he's using a cane, right? Is his kick (or arms, without it) strong enough to keep his head above water? Obviously, you can test this in a pool if not sure but waves and current are different.
Random thoughts -- can he be loosely tethered to you/a board that you are on/a boat or ski that you are on/etc.?
Can he wear floats/a vest...a way to rest if/as needed. I mean, obviously, it would be best if he's low energy to stay near shore/or swim out first thing until he's the least bit tired and then in...If he's solo, certainly wearing something reflective/easy to spot...are you usually on hand, or...? Could it be that one of the boys goes with as a lark as much as safety?
I am not sure of his abilities or the water in question. Of course, it would be common sense to make the first run in full daylight, not against a tide, etc.
I swam in the ocean during the stage I was using a walker
the hard part was getting out beyond break water. One person on each side would help and we would turn backs to waves and brace once out I didn't need help because I was strong swimmer. I welcome would use noodle while snorkeling.
coming in they would float me in like beached whale