C.H.
HI J.,
oh, I feel for you; we're in the middle of a similar experience at my house. Thankfully, the Mirilax is working for us, and my son is using the toilet more or less regularly.
At this point, I would suggest you have a long conversation with your Ped. and consider asking for a referral to a behavioral specialist. There are many reasons that kids will withhold, and both the physical and emotional aspects need to be addressed. Since he is still fighting the urge, I'd say it's time to call in some more support.
What has worked for us is to have a conversation with our son about helping his intestines work better. He knows that the Mirilax is his "poop medicine" and is eager to take it. We also use some prizes for when he has a bowel movement on the toilet. Nothing expensive: superballs, cheap party favors from Micheal's and the Party Store, stuff like that.
Best of luck to you -- this is a difficult one to survive, but you both will, and he'll go off to school in good shape! :-)
C.