Is he gaining weight? My youngest child did the same thing. I took her to the doctor because it scared me... she ate but I didn't think it was enough. She'd also sleep through when she shoulda been feeding too and you couldn't wake her up... she's fine now, healthy 8 years old... if you can, take him to his doctor just to ease your mind.
Have you discussed this with your child's pediatrician or have you taken him to a pediatric GI? If not, you should, especially if your child is losing weight or failing to gain appropriately. There are a number of physical reasons why your child has poor feeding, including hypotonia, weak suck/swallow, silent reflux, and more.