I agree with one of the users above, you definitely should build a lot of trust with him.
At least you've got plenty of work done as you've been his human companion/owner for at least two years. Still, you gotta make sure he knows you well as much as he knows you well.
Take time to share with him, have fun with him, take care of him and all. Do not try to coerce him in any way. Example: Don't give him treats to condition him to be able to get his stuff handled by you.
You could start checking his boundaries when washing him for example. Be gentle and use warm water, carefully wash his genitals and see if he likes it.
If he lets you and doesn't have issues, you can continue seeing if he likes having you handling his penis like so on regular conditions. If he's calm and all, you can continue to carefully handle him to make sure he understands what you're doing and watch his reactions.
If all goes well, maybe you can try masturbating him, again, be careful not to bother him, make sure that he's okay with it and that he's enjoying it If when you do he begins thrusting or dripping his precum you're on a good path. Make sure to give his tip plenty of love, try to emulate what a mare's vagina would feel like with your hands, maybe your thighs. If he orgasms, congrats, you've begun a sexual relationship with him.
From that, continue to explore if you wish, maybe try licking him to see if he likes oral, if so, you can continue and slowly take things until you begin using your mouth. Be careful to make sure that if you blow him, to open your mouth wide not to make him uncomfortable with your teeth. Again, if he likes it given the tightness, warmth and humidity, he'll begin thrusting and continue if he's okay with it. Just make sure to be careful when he climaxes not to injure yourself when he flares.
What's most important is to always make sure he's okay with what you're doing. Horses can and do form strong emotional bonds with humans, and if he likes this, it'll be a perfect addition to your horse's relationship with you. Seeing you not only as someone he cares for, but also as a made who can satisfy him. Make sure you take good care of him and to always prioritise his wellbeing and comfort above yours.
I hope this is useful and that you have good luck my friend.
Cheers.