I'd take him to the vet first to make sure there's no physical problem. He might need an antibiotic.
After that, he needs re-training. I'd probably use a crate and very frequent trips outside. You can also tie him to you so he has to go wherever you go and can be constantly monitored.
Oh, and you need to treat every place he has peed *extremely* thoroughly with enzyme products. I have used several for cats and so far like Nok Out the best (no odor, no stickiness, works). Cat pee is worse than dog pee, so I think Nok Out would work for you too.
Not an expert here, just what I would do......