My brother has UC he’s not doing well… I’m very concerned he looks deathly ill and very thin…

I am so concerned my brother has had ulcerative colitis for years. He’s always been in n out of the hospital and on medication. In terms of how he’s doing or what medication he’s taking I know nothing because he’s very very private about it. What I do know is that he is supposed to be on a strict diet according to my mom. But he eats the worst food for years he has never wanted to eat healthy. He will just eat a bunch of junk food and I don’t know why he does it. He will eat corn dogs with syrup, drinks that have red 40 in them, hot pockets, frozen pizza, I don’t even think he likes water…
Anyways when anyone try’s to tell him anything about his diet he’ll get so upset and blow up. Mind you we have healthy food at home. I feel like he’s killing himself at this point… what should I do…he looks deathly ill and it’s scary…