If this is the same horse as in your previous threads, this is an ongoing theme with this horse. He was alone in a field for a good while, gradually he became more and more afraid, noisey dog didn't help that (if I recall correctly). I believe (according to previous threads) the OP had finally got him a companion again, this settled the horse down a great deal and she was able to work with him again.
ALL this being said. Clearly the problem isn't fixed by having a companion. He trusts his companion, he and his friend work together to keep each other safe. Together they feel safe, apart they are afraid. OP, you're job is to make yourself equal (or even better) than his companion.
When my mare acted very similar to your horse, I started with her companion (who was smaller and easier to handle). I took him out of their paddock and walked him in a circle around the paddock. Sure enough my mare panicked, she followed us all around the perimeter of the paddock, nervous and jumpy. I did this a couple times until she no longer followed us, she had returned to her grass and just carefully watched us. So at one corner of her paddock there is another paddock near, so I walked the pony around both paddocks, which brought him just out of sight for just a couple minutes. The first time she panicked, ran around and called to him, but I just kept him walking 'laps' around that area, by the end of the second time she no longer looked up. Then I went all around the barn and the paddocks, bringing the pony out of sight and hearing distance, at this point she didn't even call. I repeated this path with him (the big one) a few times, when she no longer looked up as we passed I put the pony back. I repeated this for a few days until my mare no longer fussed when I took the pony out and no longer cared when I took him out - I eventually started taking him for several hour hikes and she was fine.
At this point it was her turn, I took her out and walked HER around all the same laps in the same sort of progression. At first they were both a bit jumpy, just kept lapping until they calmed down.
This teaches them that they and their companions can come and go and they'll always come back. Neither of my horses fuss anymore when they leave their paddocks.
What will also GREATLY help you, in your particular situation, is teaching your horse the basic giving to pressure skills. He had ought to learn how to yield his hind end, yield his shoulder, side step, back up and he should do them All on a dime, the moment you ask. He should be doing this willing and eagerly in his 'comfort zone' before you try to ask him to do these skills when he's upset or spooky. These skills sound simple, but all skills are very useful in asserting your leadership and getting his mind back on you, not whatever he thinks is scary.
I realize how difficult all this is, I went through it too. I found clicker training to massively speed up the process, but I realize that isn't for everyone. IME it just helped keep the horses' focus and kept them minding their 'rules' (particularly personal space). If you aren't into that, that's fine, but everything else I mentioned is really vital to helping this horse overcome his intense fear of being alone. That is the core of his issue - he's not scared of simple things, it's just that his flight response is seriously over reacting when he's without a companion he trusts.
That's another thing that would be very good for him to learn the 'put your head down' cue. This is cued by pulling the lead rope straight down, so there is slight pressure on the horse's poll. The result you're looking for is for their nose to drop to the ground on the tiniest tug. The reason for this is because when a horse's head is up they're in 'flight' mode when it's down they're in 'yummy mode'. It may only help for a few seconds but it certainly helps recollect a situation. I teach Every horse I know this skill, even bombproof ones, you never know when you're going to need it ;)
I am eager to hear other people's opinions and suggestions.
Good luck, I'm eager to hear updates.