Hi,
Let me just say that I am very glad that you've given your boy such a good, loving home. I have a couple of questions... first, do you know in what way he was abused? Sometimes the type of abuse can make a huge difference in the horse's mental state and what they will react to.
Also, can you give more details about what happened after you got him back after he got loose when he spooked? Is that when he stopped letting you halter him? If this is the case, I'm guessing his confidence in you as the leader has gone down because he feels like he wasn't protected then. (They sometimes forget that they are the 1,000+ pound animals ;) ) I think the only way to get over this is to introduce him to similar scary objects slowly, and show him that they will not hurt him. Let him be around them in steps, though, as it sounds like his fear is big and just leading him right up to one would not necessarily be safe. But in steps (i.e., a pickup truck) you could re-establish that trust and get him to realize that it is not so scary. I would go engine and lights off for awhile, and then slowly introduce them on.
As for the halter issue... I'd start back at square one. Spend time with him in his pasture, don't force it. I know it seems like a lot of pressure with the vet coming, but it will be better for him if you don't rush it and spend time establishing trust again. You might also look into free lunging, I have found that helps me a lot when the gelding I ride is in a spooky mood. It really helps them to focus and tune into you rather than what is "scary."
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