No interest in criticizing you :) Every child is different. It will come.
The only thing I did differently than you was to not put the child on the potty before they needed to go - but I put the child on the potty AS they were just starting to go.
I waited till they were just turned 3 - and would dedicate a long weekend to it. Off came the pants - on to the deck we'd go (or my playroom with easy clean floor) and I would watch them like a hawk. The first dribble, I'd let them feel it on their legs, and plunk down on potty. Poo was easier - they'd start to crouch or concentrate. Plunked them down.
So they never really sat for long on the potty. They were just playing away and if they started to go, I had potty on the ready.
After a few "wets" (which they did not like), they seemed to get that the feeling right before the wet sensation on legs meant they had to go. Boys too.
We stayed close to home, my husband would switch out with me, and after a few days, they'd got the basic idea. Then thick padded underwear.
Not sure if that will help you :) Good luck - it will come. And if you need to, take a break. Sometimes a month off and a kid will just get it. May not be a possibility if your preschool requires him to be trained, but lots of kids aren't - don't let that stress you too much.
Oh - as for peeing right after already going, one of mine did that. If I said "everyone go have a pee" if we were getting into van (for an outing) and my little one didn't really have to go - she'd pee, only to have to go again shortly after. We were told it was because she was forcing herself to go on demand and it kind of messed her up. She outgrew it and it's no longer a problem - but sometimes making kids go when they don't need to can do that.