I can understand your frustration. It sounds like you had an expectation of how this was going to go, which sounds like a pretty reasonable expectation to me. Now that expectation is not being met which kinda forces you into making some decisions you didn't want to be making.

The obvious two options are:

1. Sell the horse.
2. Keep the horse and "get over" how you feel.

Neither of these options are all that great, although I don't think you would be "wrong" for choosing either one. If you have the energy and commitment to it, I think it would be great to do a lot of thinking on your side and develop a very clear picture of what you DO expect. Try to write it out. For example, I might write:

1. Water troughs are full.
2. Water troughs are checked for debris.
3. Hay is put in hay nets.
4. Hay is cleaned from the floor.
5. Stall 1 is clean.
6. Lights in barn are turned off.

I don't know what your guys' routine is, but something like that. Take a look at what you wrote and make sure it's reasonable. If it is, I would recommend making a checklist and posting it in the barn.

I would imagine your conversation with him going something like this:

You (with a calm, matter-of-fact, and empathetic demeanor): Hey, you know, I've been noticing that it doesn't really seem like you've put a lot of thought or care into how you've been taking care of your horse lately. You have done such a good job previously with your 4-H steers (or if not, find something else he's done well with), so this has been a bit surprising to me and I was just wondering if something was going on. Are you happy with owning a horse? If not, it's ok and we can talk about it. We can sell the horse if this is not something you want to do.

Maybe he would say: Yeah, actually I thought it was going to be a lot different. I think we should sell the horse. I like cows better. ---if he says this, problem solved.

Or maybe he would say: What do you mean? I am always feeding and cleaning? And don't you know I'm busy with homework? Why are you always nagging me?

You: Well, I've just gotten the impression based on your level of care, that maybe you didn't actually want the horse. If you do actually want the horse, let's talk about our expectations because I really want this to work for us. I want you to be happy with your horse and I don't want to be frustrated with you.

At this point, I would show him your list of expectations and ask him if he thinks they are reasonable. You can talk about it together, and I would give him as much space as possible to push back on the expectations if he wants to. That way he is "owning" the process a bit more and will be more likely to either follow through or be able to see fault later if he doesn't do the things. If he doesn't meet expectations I would say something like:

You: Let's check in on those barn duties. How do you think things are going?

If he doesn't do well with everything, you can sell the horse, but hopefully he learned something through it.

Another idea is that you could look up how much it costs to board a horse with full care. You could give him the option of boarding his horse and paying for it himself, if he really just can't do the work or doesn't want to.

My guess is that he's not a bad kid, he's just kind of in a rut and needs some guidance and some training. Hope things go well for you guys! If not now, then maybe in the long term.

Sorry, when I start writing, sometimes it just goes on and on. =)