R.H.
There could be medical things happening. If he isn't drinking enough juices and water, it could cause him to be constipated. Thus leading to painful bowel movements. If he is getting plenty of fluids, but still not having proper bowel movements, you may need to take him to his pediatrician to make sure there is no underlying health issues.
Another suggestion (Please don't take this as being rude. As that is not what I am trying to convey.) Sometimes children will hold their bowels movements in becasue for some reason or another they are ashamed. Does he hear things like, "Ooh, that's gross." or "ooh, that stinks?" Maybe trying to nminimize these words and the negative feedback he hears, will help.
I know you said it's hard to be patient. Potting train is a treachous (chore?). It took me nearly a year to get my daughter completly trained. My son was done in about 5 months. Remember, there is no "set" or "proper" timing when it comes to potty training. Each child, even if they are in the same family, will vary. It is more important to be consistent then it is to get this done in X amount of time.
Hang in there. You are doing the right thing.