Thanks for the replies so far. I totally agree with the "don't treat him as a child" sentiment. That is why I ask here and don't make an issue of it with him. I have no intention of telling him what to do.

However, I want to be aware of signs of problems and any emergency actions that I can take if something happens. Also I don't want my ignorance to lead me to do anything that could harm him e.g suggest inappropriate foods or give cycling advice that may not be suitable to a diabetic. I might want to eat something very sweet to ward off a bonk, but maybe that would be harmful to him. He is obviously an expert on managing his diabetes day to day but he has never ridden very far and might not know the effect on his blood sugar from cycling. Any eating suggestions for cycling I might give him I want to make sure is not harmful. If he is looking very tired and close to a bonk, can he drink a sugary drink?

So signs of trouble is the same as signs of a bonk? That I can recognise since I experienced it before. Is he more likely to bonk?