The breeder should give you a diet sheet and often they will give you a little of the food they've been on too. It would be a good idea to talk to the breeder before you pick up the puppy so that you can get everything in ready. At eight weeks he will be on 4 meals a day. My pups were always on 2 milk, 2 meat but it all depends on how the breeder likes to do things. Just be sure to stick to the types of food he's used to at least for the first few months.
When I have a litter I try them on different types of food so if the owners want to change they are not upset by it, and I ask the owners what food they would find easier to get and feed. If they don't have a preference then I recommend Eukanuba complete dry puppy food till they are a year old and send them off with a supply of this. I also feed my own raw chicken wings from Sainsbury's for breakfast. Just remember to avoid the cheap coloured dry biscuit food that looks like its got vegetables in it as it is usually for of additives and colourants. Supermarket's own and Bakers are the worst.
Ooh lucky you, a cav pup, they're so sweet. I took a cavalier on at 15 months and put him on eukanuba. It upset him slightly for a couple of days but he settled down and did very well on it for a further 13 years.
Would just add that my main concern with a Cavalier is that the parents have been tested for heart conditions and syringomyelia. Check with the breeder and don't be fobbed off by saying they don't need testing because the parents are ok - you can't tell without testing!
As well as lankeela's comments, and I'm not being a merchant of doom, I'd add Episodic Falling Syndrome. It really the worried me the first time it happened and I wish I'd known at the time what it was but my dog had few and relatively mild episodes.