Could i get away with breeding a mix?

okay so i got this puppy for free! hes a pittbull/italian mastiff and i want to breed him with another pittbull or mastiff...but i have never bred before EVER! hes a fine dog pit face mastiff jaws. at 4 months old he already has muscles ripping. but idk if i could get him registered since hes a mix. i mean hes a good mix. my opinion this breed should be carried on. but i just dont know. anyone have any opinions on this topic?