Groom him, whenever my horses are off I take them to a nice green patch of grass and spend a good half hour brushing them. They love it and helps build your relationship
Try just walking him 30 minutes a day
Groom him, talk to him, do lots o beauty work if your horse likes to be pampered. But if he is not into that, go on little walks around your barn to let him graze in new places and see new things.
Teach him tricks! Also look online, the is probably a lot of things you can find.
salt licks, or we put rocks in milk jugs for them to knock around