Hi @MADS and welcome to the forum
At 79 years old, exercise isn’t really realistic even if he was healthy.
All he needs is enough to keep the circulation going, and as a diabetic, I’m sure he’s already doing that. As you’ve said, he’s already doing all the other things that can reduce the risk of a future stroke. But, as with any illness, while you can reduce the chance of having one, it’s impossible to eliminate that chance completely. If he’s already doing everything he can, then he can do no more.
Unfortunately, being the age that he is and having diabetes will increase the likelihood of a stroke no matter what you do to manage it. All you can really ask is to keep up with what he’s already doing and try not to worry about the future when it’s something you can’t control. At that age, regardless of how healthy you are, these things just happen, and stressing him and yourself out by trying to set unrealistic standards for exercise isn’t going to do anything but make you both tired and miserable.
The stroke alone can majorly affect how much energy you have, so he’s probably refusing to exercise because he’s tired. He also might not feel able to do as much as you would like him to, and constantly trying to force him to do it will only frustrate him further. A better option may be to sit down and discuss his strengths and play to that instead.
Light exercises to help regain mobility or improve circulation would be alright, but don’t force anything onto him if he feels he isn’t yet capable of doing them. Another idea is to not focus on exercise specifically, but getting him to just do activities in general and have him more active around the house. Doing things that he enjoys, or just household chores will feel like he’s accomplishing something and will feel worth doing.
It will be more productive and provide useful practice around the house to return some of the mobility and motor skills etc he had before the stroke with a tangible outcome to each task that he will want to achieve. It’s more motivating than just doing repetitive movements for an arbitrary amount of time like most exercises are and it will also better aid in his recovery. As these sorts of tasks would be things that he’s already familiar with, they’d be easier to do. It may not be as good as running on a tread mill for 20mins but at 79 he doesn’t need to do that anyway. And remember, at his age, too much can be just as detrimental to his health as not enough!
I wish nothing but the best for your dad’s recovery and take good care of yourself too.