I haven't gone through this yet as my daughter is only 18 months however my sister was the same problem with her son recently. He would go intermittently and tell her when he needed to go on some days but then on other days he'd just go up to her and say "i went potty, change me." Finally after awhile of this on/off potty usage she talked to him and told him that all the diapers in his drawer were the last ones they had. Once they were gone they were gone. Prior to this they had gone and he picked out fun little boy underwear with his favorite cartoon character on it. She made a big deal out of every time he used the potty and when the day came when they used the last diaper that was that. Every time he'd ask for a diaper she'd direct him to the old "diaper drawer" and tell him that they were all gone. She did have a few days with several accidents but he picked it up completely them within a week He was nearly 4 by the time he was trained and she had to clean up a few accidents but it worked for them.

Good luck and I'm sure it will all work out when he's ready to do it.
Kristen