Why doesn't he eat well? Give him healthy fruits and vegies and starches. Don't send him to school at this time, you are right about it. Don't put pressure either.
This happened to me too, with my younger son. I used good starchy diet and sometimes suppositories and no pressure, just kind encouragement.

I'm glad I'm not alone. I have a 6 year old who not only poops but also wets himself. He too has suffered from constipation and my ex-husband blames me for the fact that he had to be hospitalized overnight because of it. The pediatrician really had no advice for me accept to say that at some point he may have to consider seeing a psychologist. I don't believe this is necessary because my son has told me that the reason he doesn't go to the toilet is because he doesn't feel like sitting there waiting for the poop to come out so he holds it until he can't any longer. I have stressed about this for far too long and am at the point now where I just say to myself "he'll eventually stop doing this." I have tried punishing him by taking toys away but nothing seems to work. I have stressed about this for far too long and am at the point now where I just let it go.

I am dealing with the exact same thing. Not quite as long though... All I can say is, I am so sorry for you! I have a 4 year old who woke up one day and decided she was done pooping, and pretty much hasn't had a real bm in about 6 months. I've tried everything I can think of with no improvement at all. She's on Mirlax too. The Dr. told me it was just behavioral and that she would grow out of it. But I'm changing panties and clothes 3 or 5 times a day, she's in the bathroom every 20 min and still doesn't do anything. It's frustrating. If you hear anything that would help, please let me know.