Anyone is entitled to disability benefits if they meet the criteria. If diabetes is the only health problem then it depends on how it affects the person's ability to work. I doubt very much it'd be paid automatically to a diabetic- an assessment would have to be made
He MAY be entitled to attendance allowance (which is a disability benefit). However it would depend on how much help he needs with care from another person.
Well then, probably not - there are strict criteria for disability benefit (like not being able to get about and do things for yourself). there are many degrees of diabetes, many people on medication lead perfectly normal lives, just being careful. He would need to ask for himself and fill in the claim forms (which I think still requires a medical report too).
Diabetics can and do work normaly. If you inform your employer you are diabetic you are covered by the Disability Discrimination Act. Don`t know about benefits.