Mel,
I may be way off here, but just a question for you. Does your little guy eat all solids for every meal and every snack? I ask because mark went through this for years when he was younger. He told me that as he got older and more self sufficient (which was about 10-11 for him as his mom worked Full time, went to school part time, was dating and he had an out of control sometimes in rehab older brother) he started mixing his meals up.
One meal he'd have someting liquid (protein shake from his dads store-althoug I'd recommend something more along the lines of carnation instant breakfast, ensure, boost, homemade milkshake with 1/2 scoop of pure protein, pediasure if he's that young) and then the next meal he'd have a "regular" solid meal. Then for the next meal/snack he'd have something liquid again. He kind of "self medicated" (as I call it) to find what worked for him. And when he was eating all solid meals, he was having the same problems you mentioned of your little one. Since he started this type of eating habbit years ago, he's stuck with it and still does it. It seems to combat this blocked bowel problem.
Like I said, I may be way off here... but maybe an idea to try when you get this sluggish bowel all cleared up?
Hope your little one feels better soon!