I don't think there's anything stopping a type 1 diabetic age 25 from having any vaccine unless there is something else that's weakened his immune system? Diabetes can mean you take longer to fight infection but that's down to excess glucose and not your immune response in itself.
If he's on his GP's list of people who get a free flu vaccine then I am sure he will get in turn when it is due but after listening to the vaccine committee chap this morning on R4, I think they are prioritising heathcare workers and over 80s initially with diabetes getting you to the top of the age cohort list as it gets rolled out....
I believe a revised document to this effect was going to be published today following an update:
https://www.gov.uk/government/publi...cination-advice-from-the-jcvi-2-december-2020