How long has he been having stomach issues?
Stomach issues can be caused by a number of things - eating something dodgy/food poisonoing (normally wouldn't last long), food intolerance/allergy to something, IBS, some medications (including some antibiotics), probably other stuff I haven't thought of,, I wouldn't normally think that insulin would cause it, but if he has reacted badly to it I guess its always possible.
How long has he been on his current insulin - for 2 years? - if he's had a dodgy stomach all that time I'd have certainly been to the docs already.