I'm sorry to hear about your son's diagnosis. I too have Crohn's colitis. Here's my understanding at the main differences.
Crohn's Colitis affects all layers of the colon, where as UC is just the surface layers. Also, surgery is a cure for UC. Remove the colon and your remove the disease. This is not the case with Crohn's colitis. Since Crohn's can affect all the different areas of the digestive tract, removing the colon does not cure it. The Crohn's could just show up someplace else. There are things that doctor's look for to diagnose CD vs. UC. It's important to understand and make sure that your son does in fact have CD. Though the treatments and symptoms are similar, there are some complications of each that are somewhat unqiue.
There is a ton of information of both Crohn's disease and UC available online. Google away and use this site as a starting place. I'm glad to hear that adjusting the Lialda has helped. I'm on Asacol which is the same stuff but you have to take tons of pills, 3x per day. Probably not the best for a 10 year old :) but there are other options out there if he can't tolerate that medication.
As other things that help, search this board for info on probiotics which naturally help stop the D and regulate the digestive tract. Also, my GI recommended that I keep a food diary so that I can keep track of what works and doesn't work for me. It's really helped me to identify and avoid those foods that cause problems.
Best of luck.