Now this is stuff I always feel out of my depth about, so I always double check with more experienced breeders before I give advice. The advice I was told to give you is that at his age, he needs to be fed about every 30 minutes around the clock. He is likely weak and dehydrated from going so long in between
A friend of mine is going through this, she probably is one of a couple people with the most experience with rejected babies. If your on facebook I can get you two connected and she can guide you through stuff better than I can.