one thing that might help in catching him is to catch the mare first. or, bring out a bucket of treats for her (and it can be something mundane like some hay, if they consider that a "treat"), when she comes over, you allow her to have it, but you drive him off a bit. then, you allow him in a bit, and walk up to him and see if he'll let you pet him. if he turns to leave, you drive him off.
you are basically making it so that he "asks" permission to be there, and part of the deal is that he allow you to pet him. if he blows you off, then you drive him away. soon he will want in more than he wants away.
when he does allow you to pet him, make it very short. you pet a couple of times, then just turn your back, as if you are now disinterested, and walk away. after a bit of allowing him to eat, walk in again and pet him a bit, then walk off. make your petting time very short and endurable, but you decide when to start and stop.