Round penning, desensitization, ground poles, sending through gates and barrels, sending over or under tarps, yielding the front and hind end and maybe even work on side passing, liberty work is always great!
Clicker training and start slowly backing him. No weight on his back under any circumstances. Just putting the saddle pad on or a lunging surcingle just to get him used to the sensation. Maybe also some fun bonding excersises like obstacles which would help desensitize him and build trust. Also liberty is a very good one because it helps him create a strong connection with you at the same time as keeping the respect that traditional groundwork does and it helps get his mind focused on one thing at the same time as being fun.
You can do poles in different ways. Slightly raise the pulls and then trot over or put them in a semi circle