Every parent’s worst nightmare is a baby who they fear is not eating enough. If you feel that something is wrong with your baby’s eating habits and/or his weight is dropping, your first stop, as always, is with your pediatrician. You might also find a registered dietitian in your area. This site will help you find one: EatRight.org.
Your baby should be physically growing and learning skills and milestones. Is he meeting his developmental milestones? You may want to pick up a copy of this book from your local library: The Wonder years: Helping Your Baby and Young Child Successfully Negotiate The Major Developmental Milestones by the American Academy of Pediatrics. I get a lot of emails from the parents of picky eaters and it is almost always the case that they begin eating lots of food after the age of 8 months when they start moving. Is your baby crawling a lot? The more babies move, the hungrier they get and the more they should eat for all the extra energy they need.
There are some great books on picky eaters, too. Try Food Chaining, it talks about the difference between picky eaters and children who have serious eating disorders. Whatever you decide to do, make sure you keep your pediatrician involved.