Most photographers will allow an outfit change. If it were me Id do a sundress and boots for one outfit and my typical jeans, boots and a nice shirt for the other.
As for poses, etc... Can you trailer somewhere or is it going to be in your back yard? Is there a spot that is special to you? I personally love water so I would want a creek or pond or something incorporated if it were me. My point being make it meaningful too.
Some ideas for poses: having your horse lay down and laying with them. Relaxing together in the field or during a sun set or sun rise. (can be difficult though)
Laying across his back while he grazes.
Shadows can make pretty cool pictures too
I personally like in the moment shots. Posted via Mobile Device