This behaviour is very usual. The key is to go right back to basics and treat him like a new puppy. Keep all doors closed - my boys will wee in the bathroon given half the chance, pity it's not in the loo though
- keep the door shut and they don't get the chance.
Make sure you reprimand him only if you catch him doing it - and when he is outside doing his stuff - still give him lots of praise. It's easy to think coz he can do it that he should know - but they can forget and as you've found, go backwards.
Good luck