A.,
Don't worry, he is normal. Most, if not all kids go through this. Both of my sons(5&7 now)didn't potty train until they were 4yrs. Our youngest would use the potty at 2yrs and we were so excited. Then after a while, he stopped. Both of our boys were potty trained the month before attending pre-school. I put a calendar up on the wall and each time our child used the potty, I would draw a star. Our boys were very excited about this. Once they went to pre-school, peer pressure kicked in. By knowing that other boys their age were using the potty, they were willing to go. When I picked them up, I made sure they went before we left school. I kept up with the calendar as long as necessary. Their reward was going shopping with daddy for big boy underwear. Our oldest picked out the incredible Hulk underwear. He was so excited. Our youngest picked out Cars. We told them that the character wouldn't like to be peed on so try to use the potty. If they had an accident, I didn't make issue of it, I simply helped them take care of it. Our youngest is now 5 and every once in a while he has an accident. Because we don't make an issue of it, he takes care of things himself now and there is no shame or embarassment for him. At night he still needs a pull-up because he doesn't have control of his bladder yet, but he'll get there.
My suggestions that worked for us:
1)Encouraging them to go frequently. No begging, pleading or bribing. "Let's try to make bubbles in the toilet and if nothing happens, no biggie." Our boys love Kim Possible, so I use words like no biggie. They relate to this and it works.
2)Not yelling, getting angry and making an issue.
3)Using a reward like picking out their own underwear or going to the park versus a toy or candy.
Pushing them and trying to rush them when they aren't ready is a mistake as they will revert back to diapers. Then you find yourslef back to a frustrating square one.
Best wishes