After a long grooming session, I decided to work on the saddle again. He was fine when I put it on his back from both sides, so I moved on to working with the girth. I started again by just attaching it to one side. When he was fine with that, I lifted it up until it lightly touched his belly. He wasn't jumpy, but he tensed up and snorted at me, so I stood with the girth in my hand until he relaxed a little, click, then treat. I did this several times until he went back to closing his eyes and relaxing in the warm sun. I took the saddle off, brushed him more, and then we started over. The saddle went back on, the girth was attached, and this time he stood like a gentleman when I brought the girth to his belly. I was able to buckle the girth on the first hole and walk him around a little bit. Slowly I worked it up, until it was at least tight enough for him to move around in it without the saddle sliding. We played the follow game and I brushed him some more. He seemed completely comfortable.
I lunged him again, this time with the saddle and bridle, and he did really well. The saddle flaps were bouncing up and down and I thought maybe it would startle him, but the most he ever did was perk his ears and move into a big trot. He even cantered with the saddle and bridle on, and was quiet through it all.