If you are open to alternative medicine and willing to listen to their opinions, I would advocate that direction before getting a feeding tube. FOOD can be a body's worst enemy if the body is allergic and/or intolerant/sensitive to a food.
I know. I found out at the age of 40 that I have an allergy to casein (means no more cow's milk or cow's milk products), a severe intolerance to gluten and soy, and various other intolerances to about 21 other foods. All it takes is an allergy or intolerance to ONE food that, if it isn't caught and addressed (meaning, you stop eating the food that's making you sick), you will develop Leaky Gut Syndrome and, from there, your body WILL develop other problems with food that, if those problems aren't found and addressed, you will continue having problem after problem after problem, all the way down the road your entire life.
It is pretty evident, looking back on my life and how sickly I was (constipated, always sick, very picky eater, etc.) that I most likely had the allergy to casein from baby on. Then, when that one wasn't resolved, I picked up gluten, then soy, then all the others (to include both Baker's and Brewer's yeast, beef, lots of different fish and seafish, apples, bananas, I could go on and on...).
Oh! You can be allergic and/or intolerant to a specific food and absolutely crave it! Oh, yes. I remember when I was in high school, how I would literally drink myself through gallons and gallons of milk. I didn't know it at the time--I just discovered all of my food issues a couple years ago--but the body CRAVES what it is allergic or insensitive to. So just because your son is drinking lots and lots of soy milk...doesn't mean he's not horribly allergic or intolerant to it.
And I also agree with a poster that your son could have a Sensory Processing Disorder issue with his food. My daughter has SPD, and food was a major chore. Oh my goodness. Only certain things she liked, only certain flavors or textures. Only eats one thing at a time, don't ever think about mixing anything together. And bland. She doesn't like any spice or flavoring on any of her food. We're working hard on this, and she's made leaps and bounds (she's now 10). But it is a very, very hard road to hoe.
My best advice to you would be to find a good Naturopathic doctor (mine has a PhD in microbiology; he gets down to the cellular level and works on fixing the issues/problems where the root is) or Osteopathic doctor. They are trained to work with these types of issues. Along with looking into seeing if food issues might be a factor, I would also have them check your
son for heavy metal toxicity. I had severe heavy metal toxicity, as did my daughter, and we both went through chelation (about 3 or 4 times). WOW! Did THAT make a huge difference!
Also, Dr. Lucy Jane Miller, PhD, OTR, has opened the STAR Center (Sensory Therapies And Research), I think in Colorado, where they can and do work with children who have an eating/sensory issue problem. Her book, Sensational Kids: Hope and Help for Children with Sensory Processing Disorder (SPD), has been a godsend for me and my family.
One other recommendation I would like to make, if you are open to it, is to search out a medical intuitive/healer. It is incredible what they can do. I was making very good progress with my naturopathic doctor and still not getting the healing results that I needed. Finding a good medical intuitive/healer has made all the difference in my own recovery.
Good luck.