Bladder issues

Hello Vets, I need some advice on next steps for my 9 month old intact male Great Dane. He has been experiencing urinary frequency and urgency, unable to hold his urine more than two hours, often less, and frequently has accidents or can't make it outside while I'm taking him out. I took him too our vet who did a urinalysis and he had a UTI. He was on clavamox for 2 weeks but his symptoms didn't clear. We went back to our vet who took an xray and did a urine culture. The vet thought the xray showed inflammation of his prostate, no kidney stones. The urine culture showed nothing. We tried treatment with batril for a week with no improvement. After this the vet told me to take my boy to an internal medicine vet for an ultrasound. The ultrasound showed everything, including his prostate is fine, but his bladder is thickened and he has enlarged lymph nodes around his bladder. The internal medicine vet said the next step would be to biopsy the bladder. My regular vet said this would be too invasive and probably yield nothing. So he decided to try my puppy on an NSAID. He's been on it over a week and it has helped a tiny bit. But he still can't hold his bladder more than 2-4 hours and sometimes doesn't make it outside. The vet now wants to do another urine culture, but is admitting he is really stumped. Does anyone have any suggestions about where to go from here/next steps to take? I, and apparently my vet, are at a loss here.

Hello - it sounds unhelpful from a client's perspective when your vet sought the advice of a specialist and then declined to take it.  I wonder what is behind this?  'Too invasive and would probably yield nothing' is an interesting statement and I don't know the truth of it;  maybe your vet is a specialist bladder / soft tissue surgeon and has experience of this.  Maybe they don't biopsy bladders very much themselves and the idea is uncomfortable because they have never had any success with it - it's certainly not surgery that I would want to undertake in a young animal, if I could advoid it.  Would you be interested in having the biopsy taken?  If so, I wonder whether there is a surgical specialist locally, or someone who regularly biopsies bladders, who could review the case on a second opinion basis and tell you the risks and advantages involved so that you can weigh it up for yourself.