Hi M.!
I'm a pediatric nurse with ulcerative colitis myself, so I hope I can help you out.
First of all, I know how he is feeling! If I skip meds for more than a day, I start feeling uncomfortable. I found that a warm heating pad on my tummy helps A LOT. I also eat a bland, non-fibrous diet until I feel better (no raw fruits/veggies), but plenty of liquids and starches.
For the vomiting, just give him sips at a time, this will keep him from getting dehydrated. Stick with water, Gatorade or Pedialyte if he won't keep anything down, then try to give him his meds with a cracker or toast or some pretzels. He really needs something in his tummy when he takes them. He could also have Tylenol every 4 hours (not Motrin!) for the pain.
If he gets to the point where he is having bloody stools and continuous pain, he needs to see his Gastroenterologist for possibly a short course of steroids.
I sure hope he feels better soon!