make sure to clean accidents with odour neutralizing products. and when he DOES go outside, big big party when he finishes. I mean he just solved all of the worlds problems in that moment
Where is he peeing inside? is he doing it right next to you or sneaking off? How much freedom does he have?
I would greatly greatly greatly reduce his freedom. if you cannot have your eyes on him he should be crated (assuming he is properly desensitized to a crate) or a small area he will keep clean. Close doors, use baby gates or even tether him to you to ensure he cannot get away to pee.
Get him on a schedule. Feeding at set times and regular schedule overall to help his bladder fall in line with time.
Are you taking him outside offleash? I'd keep him leashed so he can't explore wihtout peeing. Do you go for walks? that's a good time to reward the pottying as they tend to potty on walks.
If he hasn't gone when he was outside and is due to go then he needs ZERO freedom in the house until you go back out and try agan. So on leash next to you wiht your eyes on him or crated (again assuming he is crate trained - you can consider crate training if you haven't already).
But again make sure to clean all accidents with odour neutralizing product such as htose found in pet stores or vinegar. Regular cleaners leave an odor the dog recognized and will make them associate that as their potty place. if there is a spot he peed often and you can't quite get the smell off (to the dogs nose not yours) then consider feeding him at that spot for the next few weeks).