You mentioned that he fell below "the" curve on his weight gain. Do you mean that "his" curve dropped off or that he is just below the average 50th percentile? Just for your peace of mind, if he is following his own curve and gaining weight at his own rate then he sounds fine, particularly since he has met all of his milestones otherwise. I know that every child is different, but my daughter has always been between the 7th and 15th percentile on the CDC's growth curves. She was was breast fed exclusively until about 6 or 7 months then started taking some pureed veggies and fruits. She is two now, still tracking her own growth curve and doing great. If this is the case with your son, you may consider getting a second opinion regarding the NG tube feeding. PM me if you want the name of a fabulous pediatrician.
Also, remember that the growth curves were developed when kids were mostly formula fed (thus bigger on average) and that we now have an epidemic of childhood obesity, so normal/average is not necessarily desirable. Good luck with everything, I know kiddo health issues can be very stressful on us parents.