Erin, your sons symptoms sound familiar to me as what happened to me when I was 25. If no one has tested for this yet, ask your doctor to test for Celiac (Celiac Disease is it's 'full' name - pronounced Sea-Lee-Ack). It's a simple bloodtest!
Basically, it's an intolerance to gluten which is found in wheat, rye, oats and barley products. It sounds alot worst than it is! So, please don't let me scare you. It's 100% treatable with just a food diet avoiding those foods.
When I was diagnosed at 25, I had very loose stools where at times, you can barely make it to the bathroom and generally soon after eating. Pleaes don't just 'try' this and see if it corrects itself because you need to have the symptoms when tested. I can give you much more info if you wish to pursue this. You can also google celiac disease on the internet - but remember, the internet sometimes makes things sound so much worst than they are.
I'm now 41 and never had any problems/issues before I was 25 and all of a sudden, I started getting sick with the loose stools. No other symptoms. It could be just a virus, however it's better to have this one bloodtest taken and know forsure if you can cross that off the list. It often appears in young kids as they experiment with trying the different foods and getting off the babyfood stuff.
Please feel free to email me further if you wish more info.
Good Luck. I hope you find your answer quickly for you and your son. It's never fun going thru that.
J.