Not a pool response, but a response from another parent.

I have twin daughters who will turn 3 in August. The were poop and pee trained at about 2 1/2, with the exception of nighttime.

But the pool is a whole 'nother variable. There is something about the pool that makes them go poop. It is even more pronounced if they come in the jacuzzi (i.e. the frequency happens more). I think it is the warm water.

Anyways, we make them sit on the potty before they go in the pool (and before we leave...they just love to go potty in the travel potty if they can!). They will almost always go pee. But if it is around the time they usually go poop (both girls have fairly regular schedules), we'll wait extra long, and usually have good success. Between that and stickers as rewards from not pooping in the pool, it has all seemed to pass.

They taught themselves how to pee over the drains in the deck. I guess better there than in the pool...

Also, the nylon reusable swim diapers, doubled up, are far more effective than the pull up ones, at least in my estimation. We found them online at some swim supply store, but I've seen them at kids stores around town as well.

What about putting a little potty on the side of the pool? Gradually move it back in the house if he starts using it?