How much to feed is going to depend on what you make, his activity level, size and just individual metabolism - so probably going to require a little bit of trial and error on your part. Most raw fed boxers consume somewere in the region of 800 grams to 1kg of raw food daily though, and homecooked is going to be similarly hydrated - so that may be a reasonable rough guide.
My suggestion would thus be to start him off on around about 800 grams of food daily, and see how that works. If he starts to get thinner, you'd need to increase a bit. And if he starts getting fatter, decrease accordingly. It's the sort of thing you need to adjust throughout his lifetime also (i.e. he's likely to need more whilst he's young and active, a bit less as he reaches middle age and his metabolism starts to slow - but that applies equally whatever food you feed).
