the EASIEST answer to this problem is to start keeping the dog on leash after his 'safe' period has passed, and you fear an accident may happen.
This prevents accidents, and also helps him learn to hold his bladder for longer.
Keep him on the leash near to you and then take him out when you are ready.
Eventually you can give increasing freedom successfully.
good luck
Cheers
Denise Herman
http://www.empireofthedog.com