This sounds VERY similar to what I went through with my mare when I got her 4 years ago.
Along with what the other posters have mentioned (and definitely getting a qualified person to help you) the things I discovered that helped my mare are: getting a better fitting saddle (the one that came with her turned out to be much too narrow), switching her bit (she's a grey so she has grey horse melanomas lining the corners of her mouth, making her unable to feel a broken mouthed bit), and as her rider, I needed to be crazy confident.
The less confident I was, the crazier she'd be. If I relax on her back and just go with whatever she does (while giving her gentle guidelines), she will calm down much faster than if I just force her to shut the behavior down.
I also did a TON of groundwork. I literally did not ride her for about 6 months after I got her. I free lunged/lunged her 3-4 times a week, brushed her, saddled her (no riding), just hung out with her. We took walks down the road together, we looked at everything, basically I pretended like she was a big dog.
After those 6 months were up and I had gained her trust on the ground, she was much better undersaddle. I've determined that she's really one of those horses that isn't just going to trust and respect a human because that human demands it. I had to earn the right of her trust (most important for her) and then her respect. She'll now respect anyone/anything I expect her to but if I'm not there, she'll go right back to being "crazy" to whoever is handling her. It weird but true.
Another thing I've found to be really helpful was getting my mare on Mare Magic (or organic crushed raspberry leaves). Obviously your guy isn't a mare but I have heard of Mare Magic taking the "edge" off geldings as well.
Anyway, if he's a gray, especially at his age, I would check the corners of his mouth. If he has the mouth melanomas the corners of his mouth will feel really hard. Most horses are very soft there but the melanomas make it hard. I know my girl was/is very worried about broken mouthed bits because they use the corner of the mouth as a "warning" spot before the joint(s) close inside the mouth.
Melanomas can take away all, or most, of the feeling in that area so the bit could be pulling a "SURPRISE! The bit is pinching your tongue and you don't know why! hahahahahaha!" thing on your boy and freaking him out.
If you find that to be the case, I can suggest some bits/bitless options that will most likely work for your boy. :)
I wish you luck. I completely understand what you're going through. On the plus side, as a hopeful note for you, my girl is now completely sane and I actually use her as a lesson horse for small children.
I thought, more than once when I first discovered her issues, that I would be lucky if she wasn't absolutely dangerous for the rest of her life and here, not even 4 years later, she's not dangerous at all.
Sassy and opinionated, yes. Dangerous, no.