My 1st son weighed 18lbs 6 oz at his 9 month checkup. He was always long and so thin. Even now at 2.5 yrs, he NEEDS adjustable waist jeans as his waist is probably 12m, but his length is almost for 3T.
Our 2nd son is 5 months and weighs 18 pounds. He's also long, just not thin. Both boys were over 8lbs at birth.
My point is they come in all different sizes. We were concerned about our 1st son being so thin. But as long as he stayed on track, the doctor was fine with it. For some extra calories, she suggested Carnation Instant Breakfasts mixed in with his milk. But, I'm not sure what age. There is also Pediasure (or something like that) that you can give him while he's on his eating strike. That should supplement the missed calories.
He'll be back eating in no time, I'm sure.