D.B.
My suggestion is that you wait. He is showing that he is nowhere near ready to really potty train. A few young kids get this early, but the vast majority absolutely do not. The novelty of using it often wears off, and their bodies/brains just really aren't their yet.
If you want to leave him naked a lot of the time at home, fine. But I wouldn't put him in regular underwear if he keeps peeing. And don't be surprised if this takes another year!
You're going to get a ton of suggestions here, and I think people mostly just stumble on something that seems to work with their kid but doesn't work with their other kids or yours. I honestly think there's very little science to this because development is so individual. Kids walk, talk, eat solid foods, sleep all night, and use the potty pretty much on their own schedule and there's not a lot you can do to change that course.