I've heard of the potato and venison, although I don't know if it was the same brand you have, but it was thru the vet office I worked for.
I think it worked very well with food allergic dogs, but you're right, it was not made specifically for giant breed dogs, but that doesn't mean he can't or shouldn't eat it. There are many sensitive stomach and allergy foods out there now. You have to find what works for you, and sometimes it can take awhile.
Keiko has allergies too, but unfortunately, they're not food allergies, so no simple changes for us. She has to be on meds until we move back home, where hopefully she will go back to normal.