8 week American Mastiff puppie, what food?

I have just brought this lil pup and not 100% sure the best dry food for him, his previous owners where feeding him puppy pedigree chum which he is on at mo, but I have heard this is too rich for him and he don't particular like it, he only eats small amounts if I mix it with puppy milk. I have researched on web but want to hear from mastiff owners as I heard if you don't get it right they can have gasteric prob's later on in life! He is booked for vets in 7 days but don't want to wait to long. Thanking you!