I'm not sure what you mean when you say your child was potty trained at 2.5 and stopped wetting the bed at 4. If he was wetting the bed, he wasn't potty trained. Or do you mean he was dry all day and just wet at night? If the latter, that's extremely common and normal.
I would definitely see a pediatric urologist. We did. Our son was nowhere near being reliably dry by day until past the age of 4, and he was still wetting at night past the age of 7. Aside from never getting a decent night's sleep (which is essential for his brain development), we were all frustrated with sheets and laundry and so on. And he couldn't go on sleepovers or really even be in a vacation rental unless we took all kinds of absorbent padding and waterproof sheets. FYI We did the alarm thing, and that was a nightmare because it only goes off after the child is already wet - we got talked into spending the money without the doctor really explaining the uselessness and downside of it.
I'd say that 2-3 urinations in 30 minutes is excessive. I'd question the blanket statement of "a small bladder" - which is what they usually say when they don't know. You've ruled out diabetes and UTI, which is a start. My guess is (which you should consider as JUST ONE possibility in talking to the P.U.) is that your son has gone through a growth spurt and has more urine, but has not really been through the developmental stage of "full bladder notifies brain to wake up." Nocturnal enuresis - nighttime bedwetting - can affect kids into their teens. Our P.U. told us he has kids as old as 18 in his practice - more often boys, but not always.
The answer for us turned out to be a simple medication called DDAVP which had no side effects and helped our son sleep through the night. He took it from 7-11, went off, began bedwetting again after 3 months or son, went back on until age 12.5 or so. By then, his body had developed enough that his natural warning signals kicked in, and he never wet the bed again. But of course it could be something else entirely, from an anatomical abnormality to something the pediatrician missed or didn't check for.
Go get a consult.