My 15 months old nephew not sitting up

My 15 months old nephew born full term but was underweight and spent 15 first days in NICU. He has detected Cyanotic congenital heart disease so his blood oxygen level is never 100% but he has never shown any cyanotic symptoms till date. his birth weight was 4 lb , and at 15 months he is weighing 13 lb. He tries to walk with assistance but he does not sit up on his own from laying position, he can roll his self. he can sit steel in his chair n on floor, and he can stand up from chair but unable to stand up while sitting on the floor. I m really worried about him. He is in India and Doctors say nothing about achieving this milestone.. Can anyone advice on this. I m really worried. plz help.

[margypops]

He may just be slow in achieving the things you are concerned about many children do not walk till this age , I can understand your concern though, he may have some delay in sitting up, I am wondering if some exercises may help him. Is he okay in other ways ,eating, sleeping is he trying to say anything ? What did the Doctors say about his weight /