I can only comment on Australia, however all the rules seem to be similar. I am a qualified and active guard and qualified Conductor. Although my work is voluntary we must meet the National Rail Authorities standards. Whilst my counterparts only do a medical once every 5 years, I have to do one every year, however 4 of those years the Dr just reviews my test results has a chat and I'm good to go.
Here is the link to the UK standards
https://www.rssb.co.uk/rgs/standards/GOOTC302 Iss 1.pdf there is no mention of Diabetes, but section -
5.1 General Health In general terms, candidates must not suffer from medical conditions, or be taking medical treatment, likely to cause:-
(a) sudden loss of consciousness
(b) impairment of awareness or concentration
(c) sudden incapacity
(d) impairment of balance or co-ordination
(e) significant limitation of mobility
(f) visual impairment of a temporary or transient nature.
(a) and (c) will be the issue so unless he has a lot of serious Hypos. A good Drs report should make a & c not be an issue