Try those pull-ups that get cold when they have pee in them. Or just randomly put him on the toilet throughout the day, he's bound to have to pee one or two of those times and he'll realize how it feels right before he goes. Good luck.
Boys just don't want to stop what they are doing, you have to be on them. My oldest will be 4 in Aug. I stopped asking him if he had to pee, and instead I would tell him to go. Don't give him the option to say no, I would just pause t.v. or whatever and announce pee time. He started running, trying to race me. Hope this helped