As long as the nappies are having soemthing I wouldnt worry about the qty, as long as somethings going in and out. Also keep an eye on his colour, do the finger test (push on the skin and colour should return almost instantly) and watch his lips and mouth are not too dry. There are lots of clues if you google which will help you monitor his condition. My boys stopped drinking at several times. 1st he was about 7wks and only have a couple hundred a day and he wasnt sick. Recently he had had a gastro bug and wouldnt have anything. Stayed at the GP for hours and learnt lots about how to check him so I felt ok being at home with him.
If you worried, could you make an appointment with your GP's nurse? They could check him over and also go through some points with you to look out for.
My Dr always tells me not to worry too much about the amount of milk he's taking, a breastfeeding mum doesnt know how much is going in, and look for all the other signs of health problems, eg, nappies, wakefullness, colour etc. And she always tells me to just call them if I'm worried and not to feel silly doing so. Better to be over caustious and be wrong then to do nothing when your guts telling you somethings not right.