There are some people who cannot float because they have. negative buoyancy. If he loves the water and has had the lessons, he knows HOW to do it. Relax a bit. You said "...he swims like a seahorse." Just because he does it differently doesn't mean he isn't doing it.
Can he push himself off the bottom to get to the surface? Does he know when he needs to breathe? Can he get from the deep end to the shallow end? Can he bring a toy from the bottom of the play area of the pool up to the top? Can he tread water and keep his head above water if he's relaxed? As long as his head is up, it doesn't matter much where his feet are.
Maybe he'd rather play tag?
With all of our running and all of our cunning, If we couldn't laugh, we would all go insane...
"Changes in Attitude, Changes in Latitude" Jimmy Buffet