Well, we have lived through this saga as well. DD is almost 6 and has just now weaned off the Miralax. I have a love/hate relationship with the miralax. I hate to give my kids medicine every day but I loved that it got her going. She would literally withhold for 7 days and then we would have to giver her a suppository or enema to get her to go. HORRIBLE and not a road you want to go down.
Does he want to go in a diaper? Would he go in a diaper if you gave him one? If he wants to go in the diaper you could do the slow transition. Let him go in the diaper in the bathroom. Then in the diaper sitting on the potty. Then you could even tape the diaper into the potty and have him go in there. I have 2 friends who have used this method and it works!!!
My dd was past the diaper stage and would not go in the diaper. Plus, at the time it would not have occured to me to put her back in the diaper. If I had it to do over again I would in a heartbeat. I read a lot about encompresis, which is the whole withhodling thing. We started regular potty times. She would sit on the potty 20 minutes after dinner time and I'd read her a book, etc. Another thing that she did that helped a ton was blowing through a straw in a cup of water. When you blow through a straw they push down on the muscles they need to have bm. DD still sometimes uses the straw cup.
Miralax really can help as well. If you can use the miralax to keep him from becoming constipated and to keep him going, you may be able to skip most of the drama. But, if he is still trying to hold it in, I might go the other route with the diaper, etc.