I could believe him being game crossed with dual purpose chicken. problem is- which dual purpose breed? He's too 'clean colored' to be out of many duals like orps, RIR, BR etc.. cross of duckwing with these breeds throw birds with much more solid colored bodies, like if RIR or buff Orp, the breast would be mostly or all red or brown/buff colored.
Welsumer is a possibility- game x welsumer definitely could produce roos just like him, but they are not exactly common breed.