I would probably start off taking him outside first thing in the morning, first thing when you get home from work. You know that he will have to do his business at these times so it should be a good time to get him used to going outdoors.
Once he gets the idea, I would try to take him out regularly so he could get used being outdoors and hopefully will catch on. I would also recommend using a single phrase for pottying and say it while he's in the act. Say it several times with lots of praise. Use this phrase only when he's to do his business.
Maggie May for some reason used to pee outside but come back into the garage and poo on the pad. She will still do it on occasion and she is 3. We have pads in the garage and in their pen.
Good luck and just be patient. It will happen.
