Stop buying expensive diapers or training pants.  Save them for when you must take him out or bedtime.  Take him shopping with you to choose "his big boy pants" explain that he is a big boy now and that he can clean himself.  He will take pride in choosing his big boy pants and the cloth will feel better than the diapers.  Be patient, children develop at different paces and ages and he will get it when he's ready.  Look for signs that your child needs to go and remind him "do you need to go to the bathroom" if he resists gently take him by the hand and tell him "let's try".  Reward his attempts and his success.  Be patient, he'll get it in time.  Best wishes!