6 day old baby

My son was born by emergency c section at 41 weeks 3 days. I am exclusively breast feeding before we left the hospital he had lost 9.7 % of his body weight. We went back to the doctors and he is at a total loss of 10.5% of his body weight. My milk just came in a couple days ago. They have advised me to feed him every three hours and give him an additional oz of pumped bm after each feed. When he eats from both breast and has the extra oz he sleeps for hours. I can not seem to wake him up and get him to feed every three hours it just does not work and I am very concerned about him loosing more weight. Anyone else have the same sort of situation?