Thank you everyone. I'm just going to focus on getting him to respect me the next couple days in the round pen, getting him to switch directions & do things when I tell him to do them. A lot of becoming the leader. Then I'll try to ride him around the paddocks (we have a track, so to speak around the paddocks... it's a small barn.) so he won't be in the arena. So if he is tired of ring work, that won't be why he balks. Also, he will have quite a few days off this week due to rain & things coming up so he'll have a little bit of time to heal & recouperate if anything is the matter with him.
One more thing: he's always had this very small bump on one of his back legs, right on the edge of his cannon on the side closest to his tail.. (not good at describing things.) Could just be how he is, or could be something weird related to this? I've tried researching it. It's not hard, it moves when I touch it. I'm confused about it. If anyone could give me any clue as to what that is that'd be awesome. I'm just really trying to do what's best for him, & if he is really in pain, I feel terrible about trying to get him to work when he's in pain & being so hard on him.