Proctitis - side effect of smelly gas from suppositories

[bcmom08]

My 6 year old son was recently diagnosed with proctitis related to IBD.  The problem was blood in his stools.  The diagnosis was made following a colonoscopy.  The GI specialists said the inflammation was limited only to the rectum and not further up the GI tract.  He was prescribed suppositories (Salofalk - 5-aminosalicylic acid) - starting with one per day.  They solved the bleeding almost immediately, but we started to notice a side effect of very bad smelling gas.  I asked the GI nurses for advice on this issue - they said they had not heard of this side effect before, even thought it is listed as a side effect of the drug.  The only suggestion they had was to cut down on lactose, that maybe he was lactose intolerant.  We have cut down lactose as much as possible, and we have also been cutting down the dosage of the suppositories as per doctor's orders in order to gradually eliminate them.  The gas has gotten a little better, but is definitely not gone.  Not sure if it is better due to the lactose reduction, or the suppository reduction.  

Does anyone have any suggestions about this?  I hope that we will be able to cut out the suppositories entirely soon, so hopefully that will eliminate the gas problem, but I'm told that this problem will likely recur over time, so I want to be better prepared next time it happens.  It doesn't make sense to me that a lactose intolerance would be triggered by the suppositories.  He did not have this gas problem before the suppositories.  I've asked pharmacists about this, and no help.  Any suggestions would be much appreciated.  I'm concerned about the possible social side effects now that he is back in school.

[boron]

Gas is probably produced by normal colonic bacteria, breaking down certain compounds in the suppositorium. Is it a sulphur smelling gas?