I can empathize with you. My son has a similar history. He also has DORV with a VSD and pulmonary stenosis and is currently 8.5 months old. He had the BT Shunt put in at 7 days old. Then, just 3 weeks ago (end of Oct), he had the Bi-Directional Glenn surgery. With the Glenn, he recovered pretty quickly -we were home one week later.
I can truly relate to you and your worries about your son's reflux. Owen has had the NG tube since he was born. I'm sure this is not what you want to hear but here's what I can tell you.
When we first came home from the hospital at 3 weeks old, he too, would only take 15-30 mL thru the bottle before getting tired. So, we would NG the rest. Feeding him was always a process and still is. We would continually try the bottle. He was getting bolus feeds during the day and a continuous feed at night. The continuous feed at night was/is his saving grace as he wouldn't normally spit this up (although a few times he did). Everyone was concerned about his weight but thankfully he retained some of the formula from the all night feed. At about 4 months old, he refused to take the bottle. We think he realized - why should I put something in my body that's going to come right back up? So, we decided that it was best just to NG feed him everything and hope that he kept it down. But he didn't. He still refluxed. It was pretty bad. He was so much happier when he wasn't eating (unlike most kids).
Is your son on any stomach medicines? Owen was on zantac and reglan after the first surgery, but it did nothing for his reflux. At about 5 months old, we started him on Prevacid only. For a while, I wasn't sure that this was working either...but I think it's stronger than the other two.
Good news is ---just after this last surgery, he hasn't refluxed at all. No one is sure why, but we're crossing our fingers that it is over.
Anyways - I hope I have been of some help to you -you're not alone. I know how hard it can be, hang in there. They say that babies will grow out of reflux at about 1 year of age. So unfortunately - time will be a factor too.