My son was on NG with 24 drip for 6 months. you DO NOT have to do a G if you don't want to. If they want to go that route get another opinion. They wanted to do it for my ds but I fought it and got him off the tube and now he eats/drinks perfectly normal!
The NG is sometimes necessary but IMHO it does cause feeding issues. They know that they don't have to suck to be satisfied. Also, what we did with my ds was to take him off the tube for a few hours. There are a few reasons he may not be taking as much, for one it doesn't feel great to swallow with that tube in their throat, two, he is getting a constant drip of formula right? he is probably never hungry. This is what we ran into with ds. Take him off for a couple hours and he could again FEEL hunger and learn how to satisfy it.
Don't always stress over the # of ounces he is getting as long as his weight is gaining. And look at how much they want him to drink by mouth in addition to what he is getting on the drip through the NG....It's most likely more than what a non-NG baby his age would take.
Take a deep breath and relax so you can be calm to fight for your son. When babies have some health issues they can be a little behind the "normal" curve and that is ok.....my ds is now 2 and right on target with everything and I can't tell you how many times they wanted a G tube or hospital for possible FTT etc. Be his advocate, you know him better than the doctor does.
Take care!