Welcome to the forum.
I am so sorry about your son suffering so much.
That sucks big time!
It's very hard to navigate the medical system when all they are prepared to give you are the big guns, big pharma, and surgery...IMO.
I am always so frustated at their pat answers: diet has no effect, or probiotics don't help, or you must try these drugs.
Just wanted to share my story with you ( I won't make it too long), basically tried prednisone did nothing, mesalamines I developed an intolerance to and even prior to the intolerance they did nothing, then my doc gave me a choice of surgery or biologics and I said NO. And then found my own way, the hard way by doing all my own research, trial and error, etc...
Here's what helped me ( and I'm hoping it might help your son):
2) slippery elm powder to coat the whole intestinal tract
3) l-glutamine amino acid powder for colonic repair
4) diet modification as in gluten and dairy free. PALEO diet.
5) bowel rest - when i was at my worst, i would just do 4-6 protein shakes a day for few days until things calmed down and it always worked. BTW there are studies out there showing bowel rest works as well as some of the big drugs temporarily. Protein shakes consisted of Mercola's Pro-Optimal whey shake in chocolate made with almond milk - the recipe i like is Morning Glory Shake, to which I add liquid minerals and liquid vitamins among other nutrients. I like this particular Mercola product only because: it's essentially PALEO, grass fed cows, no hormones or antibiotics and it contains digestive enzymes in it so I found it very soothing to my gut.
I understand the low weight issue with your son. But IMO, you have to fix the gut issues first, the symptoms first, and then you can address how to put weight back on him.
6)Maintaining good levels of vitamin d3. I make sure mine are at high 60's. I assume since you are well plugged in, you've already made sure your son's levels are high?
7) LDN, low dose naltrexone. THIS put me in remission. It is not a big pharma drug; it's cheap and has virtually no side effects.
If any of this interests you or if I can help you in any way ( feel free to email me), there are things IMO that you can control, which will have a direct impact on
your disease, the symptoms you experience and their severity.
I was able to control my UC into remission through various
supplements, probiotics, diet modification, and LDN , low dose
For more info on the protocol which has put me into remission,
please go to this link: https://www.healingwell.com/community/default.aspx?f=38&m=1666318 , and look at the 11th post.
I hope your son feels better and let us know how we can help