I think it could be a problem. My 22-month old has only gained about 5-6 ounces since her 12-month appt...and she's a little over 19 lbs. She's not even charting for weight at her age, and I doubt your nephew is either. While he may be active (and so is my daughter), you mention that he's a picky eater. Maybe he has food allergies, and he's not being a picky eater...but rather having issues with the foods he is eating so he just doesn't eat, or only eats the things he knows won't leave him feeling badly. Also, for kids with food allergies - - - eating is not something they "enjoy", especially if it always involves things that make him feel crappy. (Kids are smart, even at this young age.)
I say this because we (us and the pediatrician) were concerned over my daughter's weight and size...our gastro doctor did an endoscopy on her and found that her entire GI tract was lined with "dots" from internal allergic reactions. We had to go through all the allergy testing and found out that basically she was allergic to almost everything I was feeding her!!! (Dairy, soy, eggs, legumes/peanuts, wheat, beef, turkey, and sesame seeds!) She's ANA to legumes (green beans/peas), so this could have been a very serious situation had I forced those foods on her more. In addition, she was diagnosed with Eosinophilic Esophagitis...which should improve by avoiding all these foods she's allergic to. We'll have another endoscopy this month to check the progress of steroids and avoiding the offending foods. That being said, when you see what all my daughter is allergic to, you can see why it's difficult for her to gain weight - she mainly eats fruits, veggies and ham/chicken/pork. There's not a lot of fat/carbs in her diet.
Hopefully I didn't overwhelm you, but when I read your post it mirrored what I have gone through with my child. I hope he can get in with someone else to be examined because the sooner you isolate the issues, the sooner he'll be able to thrive like the other kids his age. Good luck!