If he's exclusively breastfed, the 'average' is about 800ml per day - so as a rough starting guide maybe 800/No of feeds. The normal range varies enormously by baby, 500-1300ml, but as you say, you've no way to know. Babies tend to take more at the breast than via bottle, too, so even if it's a bit on the low end, he can make up for it later.