My last daughter was born with two holes in her heart, she was 11 months old before this was discovered.By this time she was under weight ( 7.8 kg), unable to sit on her own, unable to crawl or stand.She was so ill it took 3 months of meds before she was strong enough for the operation.Within a month of the operation she had put on 3 kg and took her first steps.She is now 13 years old, 5' 6" tall, doing well in school, at her dance class and is a picture of health.Your little boy has had a big fight to survive, this takes it's toll, give him time, he's still very young.With time he will catch up, try not to worry.Good luck.