First of all, congratulation on going to a single action, you (he) will enjoy it very much. As to caliber, if he was leaning toward .45 colt, I would still suggest going that way. You know how it is when you want something but then go for a different one, you always regret not getting what you wanted in the first place. Another thing I think about is that the .357 is heavier because the holes aren't as big, and that DOES make a difference when carrying, especially when moving up from a J frame .38.
I must admit that ammo is a problem, way too much hording going on and causing problems for others. I know our local gun store has lost sales because they could not supply ammo for the gun the people wanted to buy. That should all catch up again shortly, the same as the magazine shortage for the AR's did. I believe most of us on Graybeard reload, and I for one, have a couple of boxes of factory ammo lying about for .45 colt that I will likely never use. And I'm sure I am not alone, so I think between a few of us we could get you enough ammo to get by until it is in the stores again, couldn't we guys? You are thinking things through well, but remember this still comes back to what YOU really want. Go with what feels good to you and what you like the looks of. 44 Man