Constipation!

Yea, I know, lovely title! But maybe someone will read this and give me some advice. Since October Logan (almost 3) has been having some poop problems. I guess he had one poop that hurt and ever since he's scared to push it out. So instead he runs and hides in the bedroom, holds onto the bed slightly bent over and crosses his legs trying so hard to hold it in. He does this over and over. So first we tried suppositories, no go, then laxatives, made his tummy hurt so then we tried an enema...that did the trick, although he really hates it! So we got the poop out. But he keeps doing it, and here it is December now and he's still doing it. How do I get him to stop? I've pulled all the tricks...bribed with treats, told him he's hurting himself, told him he's gonna have to go to the doctor, yelled, I don't know what to do to make him stop holding it in!
Any advice?