I've been a Cowboy action shooter for 14 years. If your son is over 13, he will have to use a center fire caliber. No 22's. The Ruger New Vaquero is by far the most popular pistols in use. Many also use versions of the Colt Single Action Army. 38 spl. is the most common caliber used due to it's low recoil and low cost to reload. The Marlin model 1894 and the Uberti 1873 are very popular rifles. Both rifles will need the attention of a good Cowboy gunsmith to make them sing. Rod Kibler makes good leather but going to your local match and asking around will probably get you a good used set of leather for a fraction of the cost of new.
The Georgia SASS State match is being held at the South River Gun Club near Conyers May 17-19. There will be 180+ shooters competing. Free to watch. Lots of venders too.
Good info, welcome to the ODT !