Here's an idea that works for potty training and sleeping in your own bed, etc. Hope it's an idea you haven't tried yet.
Make a small book for him. Use his name on every page. Keep it simple. Use words he is familiar with for the tube, what he calls his parents, etc. READ IT OFTEN. Here's a breakdown, fix to fit your situation.
On the front cover, put a picture of him as a baby and now. Write a title that says "_________ is growing up."
First page- have a picture of him as a newborn, maybe in the hospital with you or daddy holding him. Write "______ is born. He is a beautiful baby. Momma and Daddy love him a lot."
2nd Page- Picture of him with tube as a baby. Write "________ needs a fedding tube to help him eat and be healthy because he doesn't know how to eat by himself yet."
3rd Page- Picture of baby in bouncer or swing or something that shows he cannot sit on own or walk yet. "_________ is getting bigger, but he needs a swing/bouncer to help him sit up because he cannot walk yet."
4th Page - Picture of him at 6 mos or so- whenever he started eating on his own. " Now _____________ is starting to eat on his own. He is using his mouth and hands to feed his body and be healthy. Great job _________! Mommy and Daddy are very proud of you _________."
5th Page - Picture of your son now playing with a toy or walking or doing something he couldn't do as a baby. "Wow, look you ____________. You are walking all by yourself, you don't need your baby bouncer seat anymore! You are growing up."
6 page - Picture of son eating now. " Look at how big you are getting ________. You are eating all by yourself. You don't need the tube to be healthy anymore. You are growing and know how to feed yourself. It is time to take it out now."
7th page- However you want to end the book-" mom and dad love you ____." with a picture of all three of you.
If you read this a lot he will hopefully gently incorporate the idea of the tube is just part of growing up. It will be a way to discuss it in the least- and after the tubes are out, you could add a page at the end with him at the doctor or with it out and say something about it, to ease him.