You won't know what he may or may not pass to offspring until you breed him and see what happens. And honestly just one breeding won't tell you for sure unless you breed him to a hen with a double copy of the rose gene.. As in say a barred rock. (I use that example because the chicks would be...
Pea comb is dominant so if the hen is (P,P) then all offspring should express pea (P,p) would yeild half pea..unless what he has is a R rose instead of (r,r) in which case the offspring may contain walnuts.. I can't really tell them full extent of his comb shape though.. And without seeing...
The ideal poultry website says in red type that they are pure ameracauna although not sold as show quality. I do not know how long this has been the case but I think its as wrong to assume all hatcheries sell mutts as it is wrong to assume that all independent breeders sell top quality.