Help with puppy peeing all day

Hi,

I recently adopted my 3rd puppy. I adopted his full brother, a purebred Golden Retriever, last year and trained him in 3 days. Not a single accident in the house since he was 8 weeks old. My method was to take him outside every 15 minutes and reward only outside and never punish. He understood this very quickly. He's registered and being trained as a service dog and couldn't be better.

I adopted his brother at 6 weeks and he's now 11 weeks. I've been doing this method every day, and he knows he gets a treat when he goes outside, but doesn't hold it ever. I made a count yesterday and he peed 75 times, and not just marking, and about 40 of these were in the house. He went a substantial amount most of the time. I took him to the vet and he doesn't have a UTI and they told us to not withhold water. I now take him outside every 10 minutes, but he rarely holds it for the full 10 minutes, and I'm going insane. When I took him for a 15 minute walk yesterday, he peed 6 times, and a decent amount each time. He sleeps through the night so I know he doesn't need to go every 5 minutes.

I'm looking for any advice. I'm going insane. I just come in and out the house all day long and clean urine the rest of the time.