I would think, based on symptoms, limited in rectum, not left side. But he might have mild inflammation travelling higher...a c-scope (not sig-scope) is a good idea.
I do think 4g enemas twice daily would be good. OR...You could ask the GI to try him on a steroid foam during the day and mesalamine at night.
What oral mesalamine is your son on? It could be increased...wasn't he on a higher dosge in the past?
It's good he's not having any pain...
I consider the rectum to be part of the left side (if that wasn't clear)