How many weetabix should I feed my 35kg greyhound? (just for breakast)?

He is a greyhound, my other greyhound girl would not eat weetabix because she's a fusspot! I know I am going to get a lot of people saying dogs should be fed dog food pedigree, bakers etc. but lets get this straight, I would never feed my dogs that rubbish, full of additives, artificial colourings, flavourings etc. Both of my dogs are currently fed on Burns Holistic Dry Complete pet food with a small amount of meat or fish such as tuna, kippers, livers, mince etc. added, but I know my boy was fed weetabix with milk all his life whilst a racer and he really enjoys it. I just don't know how many I should be feeding, I gave him 6 this morning with a little milk as a trial. Any help would be appreciated. :)