Sometimes it is a long road to potty training. I've been working on Trixie (5mos old) for about 3 months now and she is about 99% trained for poop and 75% for pee. When she has an accident I just make a loud noise or say "no" firmly and then take her to the appropriate spot and say "go potty". I praise the heck out of her and give her a treat when she hits the mark.
Of course it is not always possible to catch him in the act so I would recommend keeping him confined to one area of the house, or in a pen with just enough space for a bed and his pee pad (w/ no floor showing that he could pee on), when you can't be with him. If you work outside the home, devote your days off to training him. Which sort of means trying to be with him constantly. (My pup would probably be completely trained by now if I were more consistent, though I am pretty consistent). Persistence, persistence!! He will eventually catch on, really. My older chi was about 6 months old when he stopped having accidents. But some take longer I am told.
This has been my experience, but I'm sure others here will share what has worked for them as well. Good luck.