You already answered your own question:
Ask with your long
inside leg the moment the inside the hind leg comes forward (thus when the belly contracts away from your leg).
He shall then bend as much as he is able to, for a moment.
Of course, the bend will not stay as his body is not in the right shape for the bend, probably because if the injury.
That is normal, just keep asking and ask him every step again to bend correctly. Make sure you maintain a contact on the outside and use your corners like I let you do during our lessons.
Do not forget to use your own body in the correct way to enable correct bend for Beau.
So, keep a contact on the outside rein, lift you inside rein but keep it loose. You may give a small aid with the inside rein after you asked with your inside leg, if needed, but release within seconds. Keeping contact on the inside rein will result into the horse leaning on his inside shoulder. The outside rein and your inside leg is to prevent that.
Repeat the use of the inside leg (a light sqeeze or tickle or tap) with every step for as long as needed untill the horse is straight again. This may take months!
And remember to relax your leg aid after you give it, very important.
For the rest, the circles, corners, serpentines, shoulder for, yielding and shoulder in both in riding and groundwork are all the exercises of the gymnasium to straighten our horse.
That is why we do them in the first place.
The order is this: relax your horse, then straighten and then collect.
Relaxed Beau is enough
on with the straigthening before you can do anything else.