I'll be the oddball here and say aside from a 44 Contender barrel you can go with the 45 Colt. You can start him with cowboy action loads and take it up until he reaches his tolerance. The 45 will excede what the 44 is capable of on the top end.

I won't be wronged. I won't be insulted. I won't be laid a-hand on. I don't do these things to other people, and I require the same from them. John Wayne-The Shootist