I had mitomycin but really only twice as a precaution many years ago. not a course or anything like it. my surgeon used to favour it as a standard treatment following turbt to catch any stray cells that he missed or may have been shed during the procedure (cells falling from the tumour site can "seed" to other areas of the bladder.I was told at the time the dome is a favourite spot as they float up in the water that they fill your bladder with and settle in an air bubble at the top; allegedly - I've no way of corroberating this but it sounds like it could happen!)
Its classed as a cytotoxic antibiotic which means basically it kills cells. It can be given intravenously for systemic disease but for me and your son "intravesically" ie via a fine tube into the bladder.
In my brief association with it, I didn't find it particularly bothersome but as I said, I didn't have a course so others here may be able to add to this. In fact, when I had it, it was administered, I did the usual fifteen minutes on each side and then peed it out with no obvious symptoms. I can tell you, however, that I was disease-free for nearly two years after the first dose - still my record for a tumour free period...so fingers crossed...
heres some uk info on mitomycin c that I think sums it up ok; again, ignore the intravenous stuff!
www.cancerhelp.org.uk/help/default.asp?page=4012
All the best
Tim