I know I shouldn't do it (getting into other relationship) but I really want to help, so I'll try to do my best.

I think that if horses is not such a big deal for you, or if you need to wait just a couple of more months, than no need to make a fuss out of it.

On the other hand, if you feel that horses is one of the things you like the most and that in fact you'll never enjoy them if things keep going the way they are, I think you should seriously speak about it with him.
Maybe you and him, as a couple, need to reshape your priorities until both of you get something you can both live with. Naturally, both of you will need to compromise.

Hope it's ok I'm saying it,
mic.