We have a 2 1/2 year old lab. He found us when he was about 6 months old and we had him neutered shortly thereafter (once no one claimed him). We never had trouble with accidents in the house until after the neutering. Weird.
It seems like he doesn't empty his bladder completely and when he has to go out, he has to go immediately, especially if he's excited. He manages to sleep through the night with no problems.
We started him on Proin about a year ago and it seems to help. Last week the poor guy sprained his tail...yes, I have no clue what he did, the vet gave him a shot of dex and 5 days of Deramaxx. No accidents. Sunday was his last dose and this morning, another accident.
None of the vets have a clue. He's had bloodwork done, urine cultures, etc. Nothing. Prostate checked, it's normal.
"We can judge the heart of a man by his treatment of animals." ~Immanuel Kant
Have they ultrasounded his bladder? Make sure you let your vet know that he Dex/Derramax stopped the accidents. I almost wonder if the pain relief aspect made it comfortable for him to empty his bladder completely, which makes me wonder about stones/spinal column pain, or some kind of pain (maybe from the neuter?) that keeps him from completely emptying his bladder.