Back in late August, I sent out a note on these discussion boards about constipation that is titled "DMD and Constipation." A good source.
My 3 year old has been on Miralax 1 tablespoon daily since August. He's had magnesium citrate (colon cleaner that tends to prep people for colonoscopies) 5 times since then and I have to do another one this weekend. I put in a call to his MDA physician to see how common this "clean out" frequency could be or if this is unusual.
I have not seen the other symptoms, but it sounds like worth investigating further.
Cole is 9 and during our recent visit to see Dr. Wong it was noted that Cole is literally "full of poop"! It was clearly noticeable on his spine xrays in November. I had a feeling that he was having a problem - and had just started to give him a childs dose of miralax. We've continued to give it to him since his visit a month ago and seems to be having less problems. Poor guy would be appalled at me sharing this information!
I haven't noticed the other problem you mention with swelling or changes to his skin color.
Since he is having a problem again with this I should mention that he does have frequent ingrown toenails. Anyone else have this problem?