Good bit for a flighty thoroughbred?

I have had my three year old OTTB for several months now and when i first got him he was decently green-broke. At the time he went well in a french link eggbutt and a bib racing martingale. Unfortunately he came down with a serious case of what we thought to be strangles, and i've only recently started riding lightly again. The problem is that he's now a completely different horse. He's extremely spooky and when i ask for anything more than a walk, he bolts and you have to run him into a fence The spookiness has gotten slightly better since i've been letting him smell EVERYTHING in the arena before getting on, but it's been a month of just walking for us and the problem isn't any better. I've had the vet out, tried several different saddles, several different bits, tried lunging before i get on, switched his feed to something without molasses or alfalfa, and i've been working with a trainer lately as well. NOTHING IS WORKING!!! I can't help but to think he's uncomfortable somewhere but as of now i don't know why he would be since he was fine a couple months ago. So i have come back to the bit problem. I've tried his regular eggbutt french link, i've tried a loose ring french link, a full cheek slow twist, a comfort snaffle with a barrel, no bit at all, and he still insists on throwing his head down against the reins, even when we are just walking around without contact, on the buckle, or taking off and running through it, my trainer and i are running out of ideas so to speak lol. So any suggestions on something he might find more to his liking? Also i'm no longer using the martingale, do you think this might make that much of a difference?