I feel sorry for that poor animal. It's one of those situations where you have to feel sorry for the poor animal because he's gotten that way only because of how he was treated and handled by us at one point.
I have never dealt with a horse that has that issue yet but here is what I would do. Having him tied, and having someone he likes standing next to him, I would give him some sweet feed, or something that he will view as a treat. Put the saddle on and ever so slowly tighten the girth. I mean by that, do a little at a time.
The purpose of this is not for him to be tacked up and for a ride (in fact do this only for the excercise and don't ride him afterwards)but to teach him that he can re-learn to be ok with it, that in the right hands he will see getting tacked up is not as bad as he was once shown.
Not sure how much time you have to do this, but a period of 1-2 or longer, whatever it takes for time and with patience you can teach him it's not all that bad.
As I said tho, I have never taught a horse it's ok to get a girth put on. Do it VERY slowly and only baby steps at a time. Talk to him, pat him, teach him he can trust you. Something like this will probably take a few weeks of rehab back into something that is ok for him.
When I had to teach Cobalt getting caught was ok, I would catch him, take him out of his field, tied him then feed him some sweet feed. While he ate I brushed him, petted him, talked to him. With time and lots of patience and loving on my part, he realised that being caught was A-OK. He now comes to the gate when you call him and he'll put his own head in the halter if you open it in front of him
There are no problem horses, only problem riders/handlers (not specifying this to you obviously as you are the fixer in this situation).