Hello, Angharad.
I'm 71 & have been diabetic 10 years. I still play a good game of tennis & table tennis.
At 50, with no specific health problems, I was inactive & 2 st overweight, couldn't run for a bus, & felt I was approaching late middle age. I'm a lot fitter now. At that point I joined a tennis club - I felt that I had to take up an activity I enjoyed & could maintain. Try to do a physical & other activities together.
With care, sensible diet & physical activity, he should stay healthy. It will be important to note any signs of "ageing" & NOT accept them as signs of ageing but as conditions to be tackled. Conditions like sleepiness, thirst, aches & pains may be signs of treatable conditions.