We never had to deal with the feeding tube issue, but we did struggle to maintain DS's weight. He did have an ng tube in the NICU briefly. He ended up "topping off" and hurled it all up. One thing we did was switch from predigested formula (pregestimil) to regular infamil lipil. We knew he'd drink that, the stuff they wanted him on, he'd clamp his mouth shut, and refuse to eat. The nurses & dietician at the NICU seemed to think we should force feed him, which just made matters worse.
Also, in order to get more calories, since we had issues getting DS to drink enough volume, we were given a recipe for more powdered formula to liquid ratio. I think he was on 27 kcal formula for most of his first year. As DS got older we added a tbsp of cream to his bottles. This was as per our doctor's instructions. Once DS turned 6 months old, we were also told to give DS regular food NOT babyfood. Grind up what we were eating. 'cuz baby food is low in sodium and fat. He basically ate a LOT of yogurt. I did end up giving him baby food, too (mac & cheese, oatmeal w/pears, meats & veggies), 'cuz he had texture issues, but would add butter and salt to them.