I'm glad you think you know who the antagonist is. On further reflection, could it be his history? The history of his father abandoning him and him abandoning his wife? Those are significant factors that really impact him mentally. Though supposedly a big part of his problem is thinking life is meaningless. Where does this come from, exactly? Maybe just apathy because he doesn't have anyone in his life besides his horse? I'm not sure.

1. He agrees to deliver the message in the first place because the man is dying and the MC isn't heartless and thought the recipient was in or near the town. He thought it would be a quick errand that would be somewhere between a few minutes and a couple of hours, tops. When he discovers how big a deal it's going to be, he tries to back out, but is talked into delivering it again by a spiritual being who he at first thinks is a real kid. At the end, we find out (or figure out, I may not make it obvious) s/he's the unborn baby of the pregnant woman (he doesn't know she's pregnant) he's delivering the message to. Near the end, his little kid spiritual guide is hurrying him because she's in labor. In general, he's not officially running from something, but he definitely has ghosts in his past. I wouldn't go so far as to call them demons, but he carries a lot of guilt about his wife and child. He ultimately agrees to deliver the message (still not realizing the FULL cost until he's on the journey) because he's generally a good person, he got talked into it by the kid, and he wore out his welcome in the town he'd been staying at the last couple of months because of his drinking and arguing.

2. It's a coincidence when he finds his dad, though this question brought up the idea that maybe the kid will guide him to his dad in order to help him reconcile that part of his past. Thanks for that idea! He doesn't recognize his dad at first, but his dad recognizes him. Though I'm now thinking (thanks to you asking questions) that maybe the dad is scared to tell him the truth so he doesn't and the MC simply stumbles on something that lets him know who this man is. Thanks for that!

3. I think the climax is when he's beaten down, but decides to continue on anyway. I reread my notes and at one point, he wants to quit and the kid asks him if his leg is broken and through a series of questions, he realizes he has no real reason to quit. He just wants to quit because it's hard. But then later, maybe his leg really is broken, but he decides to continue on anyway because he agreed to deliver the message and even though it makes no sense to him, he's a man of his word. (That might harken back to something in a conversation between him and his dad--thanks again!) Though I think the healing starts when he decides to save his dad. Maybe that's how he ends up breaking his leg and when he decides to continue this journey anyway?

He finds out his wife and child died long before the story starts. That was actually what I wrote about in the story about him. I know you said to make him likeable and I didn't do that. But that was such an important part of his history that drives so much of what's going on with him in the story, that I felt I needed to flesh that out more. He had decided to go back to her and knocks on the door, only to find other people living there. He was gone about a year and at first thought she had a new man, but the guy tells him the husband (not knowing who he was talking to) was as worthless as a rifle without a stock and it's because of him that the woman and child died because he wasn't there to get them help when they needed it. This causes the character to feel a tremendous amount of guilt and regret and to feel worthless. He had told her he wasn't a one-woman man so he's not been in a relationship since (now several years), which is why it's a subtlely big moment for him to decide to stay with the recipient who is likely the widow of the man who died. Though she'd already gotten a telegram so she knew. There are a few plot holes like why didn't he send a telegram? lol

Anyway, does this help you figure out my antagonist? It helped me figure some things out, so thanks for that!