The book is called, "Breaking the Vicious Cycle, Intestinal Health Through Diet" by Elaine Gottschall. It explains the Specific Carbohydrate Diet. The basic premise is that people with IBD and other diseases cannot digest/absorb/process MULTIPLE-SUGAR moleculed foods. So, those are eliminated and only SINGLE-SUGAR moleculed foods are allowed. This allows the body to properly digest the food that is taken in. When complex carbs (multiple sugared) are taken in, the gut can't digest it, they stay and ferment causing irritation, inflammation, illness. Many people find that adhering to the diet with absolute strictness heals their gut. In fact, some people find that after doing the diet strictly for 1-2 years, they can add regular foods back in and eat "normally" again and not get sick.
Here is a weblink to get more info:
This link will give you info about the book, to how to get started, or where to find online support groups for people who are doing the diet.
I'm not a doctor, so maybe I am WAY off base to even suggest this - but maybe the pentasa IS giving him diarrhea. It's a mesalamine drug and I know that it can cause diarrhea in infants whose mothers are taking the medicine and breastfeeding. It might not be a far stretch to think that since he is so little, it could have a similar effect on him. BUT, again, I am not a doctor, just thinking out loud.
If he's hungry, I would consider the diet. I did it years ago and just started it again last week because I was flaring pretty badly. Five days later, I'm significantly better. That's why I wanted to suggest it to you. You have nothing to lose and hopefully everything to gain. Keep us posted on your little guy.