Diabetes would have been diagnosed from an HbA1C test and you/he should have been told what the result was, the threshold is anything over 47mmo/mol but depending on how far he is over that there may not be much work to be done. Diet is probably more important than exercise but if he can just do what he can anything will help.
Metformin alone is unlikely to be sufficient without reducing the amount of carbohydrates in his diet so watching the portion size of bread, potatoes, rice, pasta as well as the obvious cakes. biscuits, pastry and sugary drinks and basing meals on meat, fish, eggs, cheese, dairy and vegetables and salads with fruit like berries rather than high carb bananas, grapes, pineapple and reduce any snacks.
Have a look at this link as it may be helpful for you to pick out meals which would be suitable that he likes.
https://lowcarbfreshwell.com/
It is based on a suggested no more than 130g carbs not just sugar per day.