If you suspect he is diabetic the first thing to do would be to test him. This can be done using urine test strips which in the UK can be bought at pharmacies or on amazon. There's more info on testing here:
Diabetes in Hamsters - Vectis Hamstery . If he is unwell, I would suggest getting him to a vet even without testing as vets can do it too. I wouldn't go by smell alone. I have some hams with sweet smelling urine who have persistently tested negative for diabetes.
I would agree with cutting out the fruits and sweet treats. Pure winter whites are not that prone to diabetes, but hybrids are.