I just went through a similar situation with my daughter, who just turned 15 months old. She wouldn't eat or drink, very sleepy, etc. My pediatrician was concerned that she was dehydrated, and when they checked her blood levels she in fact was beginning to dehydrate. We checked in to the hospital for an IV rehydration, hoping that getting the fluids she needed would help her out. Sure enough, the next morning after a full night on the IV, she started being her old self again. It was so hard though, watching my little girl with an IV, just miserable. But very thankful that it ended up being a minor thing. When they're so young, they don't understand that if they don't eat or drink, we need to make the eat or drink, even if it's from an IV.
Hope that helps, hope he gets better soon!
: ) Becky