We have had the eating battles with my daughter as well. Have you seen a behavioral psychologist at the children's hospital who works with eating disorders? They had some good advice for us. They would watch me feed my daughter and then they would give me new techniques after the meal was over. Also, there is one book that I really thought did a good job addressing feeding children. It is called "Child of Mine: Feeding With Love and Good Sense" by Ellyn Satter. We also put my daughter on cyproheptadine. It is a decongestant and the side effect is hunger. It has helped my daughter.
My daughter was also on a feeding tube when she was four months old. She was on the NG tube for three months before we found out she was refusing to eat due to a milk allergy. It was a life saver for us during those months.
My daughter is now three and a half. It is just during the past four or five months that she has turned the corner in respect to her eating habits. She now eats what I ask her to!! I still have to feed her most of her food that she doesn't enjoy, but we have made a lot of progress. Not sure what the change is, but she is now at an age where we can discuss the importance of eating and she seems to understand. She has gained the weight she has needed to finally, so there is no more discussion of a feeding tube.
Maybe you could just look into night feeding, so he will still eat during the day. From what you describe it could be a power thing going on with your son (probably cf related too). They tend to know how desperate we are to get food down them and they want to be the ones in charge. From what I have learned from my daughter is that they always will win the food battle and the less I stress and concern myself with their food intake the better they do (hard to do sometimes). I also have a 17 month old with cf. I left the feeding up to him starting gradually at 8 months and now he won't let me put a bite in his mouth. I am very happy about that. It is all up to him and he is doing great with his weight. I think the Prevacid my children take also helps.
Sharon, mom of Sophia, 3 and Jack, 17 months both with cf