Good on the D3. Seems like everybody with CH is low -- in fact, most of the population is low. If I'm remembering right, Batch's target is 85 (you should not take my word for that, though).
So, if you have access to an unlimited supply of E tanks at $10/replacement, I think I would just get a 25 lpm 870 regulator overnighted so he has O2 in the short run, unless, as I've said, he's doing so much better that it's not urgent. As Jeebs has said, the prednisone can clear up a CH cycle, or reduce the severity of one, or the CH can come back with a vengeance once the taper is ended. As we've discussed, if you are the one who has to transport the tanks, an MM will be a whole different ballgame than an E.