REPLY:  Break this problem down into smaller baby steps, using advance/retreat. Let me explain. Since you've ruled out pain/poor saddle fit, let's assume this is a past sensory memory fear then. When you flush out a fear like that, you are going to use advance/retreat to get them past the fear issue. Here's how: Bring the saddle close to the horse, but then retreat. Figure out where that "bubble space line" is where you see the horse tense up about you holding the saddle near him, and do dozens of repetitions of you advancing, staying just this side of that bubble space tension line, then retreating. Over and over again until you see the horse start to relax about just that one stage. Pause a few seconds after every retreat, away from the horse, to give the horse time to digest that retreat/success.

Now advance just one step over that "bubble space" line and then retreat away (you are holding the saddle in your hands throughout this). Do that dozens of times. Soon the the line is redrawn and there's a new "bubble space" line, but closer to the horse. Advance/retreat, but try to retreat, then pause, before the horse has time to tense up. It takes as long as it takes.

Soon you'll be closer to the horse. Touch him on the side with the saddle, but instantly retreat (would help if you had someone else holding the horse in halter/lead throughout all this, so the horse doesn't wander away during this desensitizing lesson). Pause. Then repeat. Once the horse can handle you touching his side with the saddle and you retreating, and he's more relaxed about it, now approach and raise the saddle up a little higher and touch him higher up there with the saddle, but instantly retreat. Do this dozens of times until he's completely relaxed about that.

Now, switch around to the other side of the horse and start all over and repeat this entire exercise so he gets desensitized on both sides equally. Advance/retreat. It's during the retreat that the horse gains confidence with something they fear, so be very retreat focused there, not goal focused.

Pretty soon being touched on both sides with the saddle raised up high is not going to be a big deal to him IF you've done enough quick retreats there. Retreat, retreat! Can't do too many retreats in my opinion!

You might end the lesson on that point for the day if the horse surmounted just that, and let it soak for a day or two. However, if he's doing really well with all that, you can move on if you feel he's ready.

The next step is to gently raise the saddle onto the horse's back, but just as quickly remove it and walk away/retreat. Pause a few seconds, your back or side to him (no eye-to-eye contact during the retreat phase) so he digests his success there. Do this stage many, many times. Again, be retreat focused, do dozens of times of putting saddle on, taking off fast, retreat. Do it on both sides of the horse.

After the horse handles that well with complete relaxation, both sides, now set the saddle on and leave it on for 5-10 seconds, then remove, retreat. Many times. Do equally on both sides.

Once the horse is okay with that, now leave the saddle on for 20 seconds, then remove/retreat. Pause. Repeat. And so on. Soon you'll be up to a minute of the horse quietly handling the saddle on the back. Might be a good place to end the lesson for the day to allow the ultimate release of pressure for "right answer." And let that lesson soak.

Keep working on the problem like I show above: advance/retreat, adding more & more minutes to the time the saddle rests on his back before removing/retreating with the horse relaxed about it, and soon the horse is going to get past this problem altogether.

I would then work on desensitizing the horse to fear of the stirrups like I show in my DVD set. You should have already accomplished the moving of all four quarters with just your hands, like I teach in my DVD, so the stirrups now shouldn't be as big a deal, but start off very softly there with the stirrups and show the horse that you're just asking for individual quarters just like you did earlier with your hands, but now with the stirrup. If he has problems with your hands alone doing that, you need to back up and work on just that for a while.

I wanted to pass to you a link on my web site that talks a little more about saddle desensitizing in case you didn't see it earlier:

Saddle Fears - Horse fears saddle after saddle slip incident

Work on advancing/retreating, breaking this down into better baby steps and you should be able to get him past this fear. Again, I can't emphasize enough: don't be goal focused there, be: retreat focused! Hope this helps!