What type of exercise and stretches are good for a 36 year old quarter horse?

Look up carrot stretches
Look up horse stretches, and just about any of them will be good. His muscles will have tightened over time, so you may want to stretch after a bit of a warm up and after work so that his muscles are warm and a bit more limber already.
In terms of exercise, I'd say the general work of most retired horses. Not asking too much of him, just light trot work, maybe just putting him on a lunge so he doesn't always have a person on his back as 36 is very old.
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