Yes, I think it might be a good idea to try feeding him once a day for the reasons you mention.
His system would not have to work as hard to digest one meal as it would with two meals a day. so as you said, his system would get less of a workout with one meal.
It's worth a try.
Also green tripe has a lot of good stuff in it for the digestion. If you can get some frozen green tripe, you might give him some every other day for awhile and see how he does.
It's all sort of trial and error at this point. Do you know exactly what he has? IBD, SIBO, EPI, all those are very similar.
That is good news he is doing better on the prednisone and I hope he continues to improve.