RE Davis , Dunlop, Cabin Creek, etc all have 20 bore kits . Chambers have 3 that I know of from a 41 in barrel to 46 in barrels. Then there are the North West /trade gun kits, or the French fusil fin etc . You can see some of those on the Track of the Wolf site
Take a look and decide what style of gun he's interested in, and that will narrow the choice
down as far as whose kit . Personally, my favourite 20 bore is an Officer's Fusil kit from Chambers. It is the best handling 20 bore gun I've ever had.