Yes, there are different 'rules' on blood sugar levels depending on age and medication taken.
The main problem with high blood glucose is long term complications. Those complications take a long time to develop, so running on the higher side as a 36 year old is not the same as running higher as an 86 year old, the 86 year old won't have another 50 years of damaging BG where the 36 year old may have 60 more years.
There is also the risk of hypos depending on medication. Hypos can easily lead to falls, which in older people can be life threatening, so I can understand doctors being hesitant to switch him to medication that can cause hypos.
Does he still have an infection going on? Is he taking medication for this, such as antibiotics or steroids such as prednisolone or dexamethasone? Both a lingering infection and the medication could increase his BG.
he has a good diet but nothing brings the level down
What does he typically eat?
There may be room to adjust some of his foods to lower his numbers.
It may be worth getting a free CGM trial to get a good idea on how different meals affect his BG so you can adjust the meals that most affect him:
https://www.freestyle.abbott/uk-en/getting-started/sampling.html