It’s hard letting go isn’t it, especially when they have medical issues! But they have to grow up and learn to deal with things themselves. I’m sure he’ll be fine.
My daughter went on a school trip to France when she was 12, we had a meeting with the teachers to explain everything that was required but she had to learn to do her pump refills by herself, which she’d never done before. I tried to write a step by step set of instructions but she didn’t want them and preferred to commit them to memory. She managed amazingly well and I didn’t get any emergency phone calls, I think the teachers with her were more nervous than either she or I were!
Tips for travelling: work out how much of everything you need (insulin, needles, lancets, test strips, pump consumables if on a pump) and take at least twice as much as that. Don’t pack insulin in your suitcase if flying as it will freeze in the hold and then be rendered useless. Pumps and sensors may be unable to go through body scanners, you can ask to be searched with the hand scanners instead as they are safe (as are the metal detector archways). Take a copy of your prescription and/or a doctors note explaining that you have diabetes just in case you get any jobsworths at the airport asking about any equipment, most airport staff see plenty of diabetics every day though and are used to it.
I bet your grandson will have a whale of a time and manage everything perfectly well! And then once he’s done it the first time you won’t worry so much the next time 🙂