my 8 week old son has had low weight gains and ...

my 8 week old son has had low weight gains and as a result I have had to take him to a paediatrician, who has advised me to top each each 20 min b/f with 100 mls of either formula or breastmilk. (I got the blame for not producing enough breastmilk, I also got told to reduce the length of the b/f from 45 mins to 20 mins) The problem that I have is that my son before this was exclusively breastfed and as I want to minimise how much I use the formula I am expressing milk inbetween feeds. I am struggling to get out enough breastmilk to provide the entire top ups. As he is being fed 6 feeds a day I am lucky if I can express out 180 mls in a 24 hr period. To cut a long story short I was just wondering what sort of quantities other people were able to express out at 8 weeks. I know the amount varies for different people but your info would be helpful. [Edited on 16/02/2009]