my 3.5 yr old son keeping going to the bathroom on the floor

My Son is 3 1/2, and he keeps going to the bathroom on the floor. He does use the toilet at times, but obviously not all the time. I dont know what to do. I dont know if i should punish him. He needs to know that its not okay. We thought about giving him a reward for using the toilet at all times. But then again i dont want him to think that he gets something everytime he goes to the bathroom.

[AnnieBrooke]

For Pete's sake, give him the reward.  Hang a calendar in the bathroom and buy some stick-on stars.  Tell him, every 15 to 30 minutes, it's time to get on the potty now, and when he does, he gets a star on the calendar even if he doesn't pee.  Pretty soon he'll get it, and after a while the stars will lose their pull (but they'll have allure to him for a while).  He'll get it.  Either that, or if it's too much work, put him into pullups.  

[mommytash]

I have tried the Sticker thing... am still trying it! He does wear pull-ups and he pulls them down and goes on the floor. I ask him about every 30 minutes if he has to use the toilet, he tells me NO then shortly after that he pulls down his pants and goes on the floor. Sometimes he tells me he has to Poo then i take him in there, and he says he doesnt have to anymore
(like he is afraid of the toilet) but then he still goes on the floor. Thank you for your Input though