Bowel Movements: Toddler, Luv

HELP!!! 4 Year Old BOY Holding Bowel Movements for DAYS!!

I have a little boy who is almost four who just absolutely will not have a bowel movement on the potty. We have tried toys, food rewards, stickers, everything you can imagine. We started training him a few weeks ago, and he was doing great for #1, but I kept noticing he wasn't having bowel movements (didn't have one for 5 days!!), and before we started this he was having BM's every day. Background on him: He has also been developmentally delayed and is shy, can be stubborn, says he's scared about pooping in the potty, etc. I know boys...