Sarah,
Sweetie..take a deep breath. It's okay that he's not a good eater. Now, my scale at home weighs my daughter differently than they do at the office. If you think he's losing weight, then maybe you can go back to the doctor and weigh him or seek advice from the doctor?
My daughter is 13 months and she's barely touching 18 lbs at the moment. She barely eats solids...but she's fine. She's gaining a little bit here and there.
Babies are more in tune with their hunger than we are. They know when they're hungry and they'll eat.
More importantly, is he acting hungry? Is he having proper bottles? Does he wake at night hungry? Until 1 year, all their nutrition comes from their bottle. I don't know if that eases your mind a little, but it did for me. If babies catch on quicker, then great..you can start making solids more of a main source of food but you really don't have to worry that he's not eating solids right now.
Big hugs, Sarah...you sound so worried.