My son was full-term but has always been at the 3rd to 5th percentile. Our ped has been happy as long as he is gaining consistently and staying in that range. A couple of things we have done: Stay on whole milk. Add malted milk and/or Nestle Quik/Ovaltine to the milk. There are some lower sugar versions out there now. Also, we kept juice to a minimum ... to try to keep his appetite up. Cheese and peanutbutter were two other things our ped suggested. PB on apples or celery works for us sometimes. Finally, as long as he is growing, healthy and hitting his other milestones, I've really stopped worrying. When my mom visits she trys to feed him constantly and I just won't do that. He eats when he is hungry. Hang in there.