If he's not sore with the lighter work you do, then just stay there for another month or so while he's also getting hands on help to help with his issues. When you start to increase the work, start very slowly adding either time, or an increase in intensity, and never increase both at once.
I would nix pole work during the ridden sessions for now. Get a solid 30 minutes of forward marching walk, the start adding in a little trot and reduce the walking by the same time (increase in intensity, not duration).
Then keep the same amount of trot and add back the walking time you lost (increase in duration, not intensity).
At that point, the trot work can increase by a minute or so every 2-3 days, which will increase the total duration by that much.
At 45 minutes of total time, of which 10-15 is trotting, once you've been there for a couple weeks, then start thinking about short stints of canter - maybe literally just down the long side, trot the short side, canter the long side, then switch to the other lead and do the same - that little. The goal is obviously to avoid being sore, so start any changes you make in very small increments.
At 45 minutes of w/t/c, THEN I'd start back with some walking poles during the ridden work. Otherwise, take the 1-2 non-riding days and incorporate some walking pole work
______________________________ The CoTH CYA - please consult w/your veterinarian under any and all circumstances. - ET
In addition to the above, ask the chiro or a massage therapist for pre & post work stretches. I do a few minutes with my guys and think it helps a lot. You can also ask them about ice vs. heat. It might help to cold pack the sore area immediately after a ride to reduce inflammation but I could also see heat to improve circulation/relax muscle. Since I can rationalize either one, definitely ask a pro. Whatever they say, you don't need to go ultrafancy- just get a big beach towel, soak in the appropriate temp of water & lay it on him.
The exercises from the book "Activate Your Horse's Core" may be helpful to begin and gradually increase as a way to stabilize his core/pelvis muscles as he comes back into work. They incorporate stretching and strength training.
The video shows just one but the book has several - particularly the lateral bending exercises that would likely be helpful.