16 year old not gaining weight due to fundoplycation

[Triwendy]

My son is 16 and at 2 months old (premie) he has a fundoplycation due to severe acid reflux. With all of this he never has been able to gain weight, the nutritionist says we need to give him as much high fat and high calorie diet as we can. He eats any where between 7000-10000 calories per day and he can’t even gain 1 lb a week. The last gaseo we went to did a full blood work up due to heart palpitations and his cholesterol was only 117 and he stated he is not maintaining any calories in his body because he has instant diarrhea (15 min after he eats) he is now 6’ and weighs 138lbs. The only reason he is as tall as he is, is because he was on growth hormones due to small stature. Feel like we are kind of at a stand still because not sure how to put weight on him. Thoughts?