4 year old son won't toilet train

My 4 year old son will not toilet train, he knows when he needs too pee and at times will but he seems to be having a hard time moving his bowels.  I really don't think he is constipated, he seems to be holding it in and many times a day will go sit on the floor, and do his best to hold most of it in, the doc figured constipation because his belly gets hard and such, changed diet, tried laxitives but nothing helped.  I have been paying close attention to pooing times and he is trying to hold it in with all his might.  I think that they may be too hard and hurt and this is why he can't relax and holds it in.  He is now getting embarassed about wearing diapers around most people but wants it on, I took diapers from him but in the span of a day he may squeeze out more than 10 small movements...he basically holds it in as long as he can and his belly gets harder and he loses his appitie and for a few days he slowly lets a bit out at a time.  We could be walking to the store and he will sit on the sidewalk every few minutes and at home same thing.  He doesn't like people seeing him and tends to go out of sight and if someone notices or walks his way I say he is having his time and doesn't like to be bothered.  It seems to make him feel good.  My biggest concern is the fact that he is embarassed by others seeing him with a diaper on and he has been saying I am a big boy right mom...I tell him that he should be wearing big boy underware and encourage him to try to poo on the potty, offered rewards, did the pee dance etc...but he now just refuses to try.  He has been complaing more about his belly being sore, I tell him its because he holds in his movements and tell him to try to let it all out but he won't...I am worried that its just going to get worse, he is possibly posining himself by holding it in, along with his self esteem and such if it persists.  As time goes on and the longer he holds it in the smell is really bad...and because he does it so often the poor guy smells like poo hourly and at times refuses to stop playing right away to be changed.