Potty Talk

WARNING: This post is about personal bodily functions, if you are offended by references to these then please refrain from reading this post!
Liam has been Potty Training now for I think about 4 weeks. He has mastered peeing in the potty and rarely has an accident. However with that said he has yet to master having a bowel movement in the potty. He has been withholding his BM's and will go 2-3 days until his body finally can't help it and he does a BM in his undies. I have added the recommended 8 grams of fiber to his diet, increased his fluid intake and still he is constipated. We have researched this online and have done all that has been suggested and yet he is still withholding. Does anyone out there have any suggestions? I am a desperate momma who wants to help my son, any advice would be greatly appreciated!