First off, never punish! For potty-training it can be the worst thing you can do. Rewards are always good. Also keep in mind that it takes a lot longer to potty train boys than girls. I have a brilliant nephew who will be four in a couple of weeks. He can read at a second grade level, but he just barely got consistently potty-trained a couple of months ago.
Have you tried buying his own potty, letting him pick out his own boxers/underwear? Make it a fun experience and he'll be more willing to be consistant. Also, you said you have tried rewards, have you tried a potty chart? This has been proven to work time and time again.
You can reward your child with stickers and it gives a visual reinforcement if you put it somewhere where he will be able to see it. You put a sticker if he goes correctly, and you don't give him one if he doesn't, but just tell him simple, that's ok - we'll do it next time right?
Here's an example of a potty chart - you can make one or print them off the internet!
http://www.pottytrainingconcepts.com/images/Potty-Chart-DORA.jpg

