Hi Beki,
Your son's probably picked up a mild gastrointestinal virus, which should run it's course in a few days. Rice, apple sauce, bananas, and oatmeal can help to bind him up a bit, but don't overdo it or he can become constipated. Avoid giving him high-fiber fruits such as plums, apricots, and peaches until his bowels firm up again, and avoid fruit juices. Keep him hydrated (which you're already doing). It's a good sign that he's not irritated or acting differently.
If the diarrhea does not clear up by the weekend, you should schedule a visit to his pediatrician. Since you're probably introducing new foods at this age, it's possible that you've introduced a food that he's intolerant to (not necessarily allergic), and the loose bowels could be an indication of intestinal inflammation. If this is possible, back off of new foods for a few days until the diarrhea clears up, then reintroduce foods one at a time for 1 - 2 weeks to see if anything causes loose bowels again. Common culprits of food intolerance and/or allergy in children are wheat, corn, soy, dairy, tree nuts, peanuts, and fish.
BTW, you haven't given him any honey, have you? You should still be avoiding honey until after the first year, since it can cause botulism in the very young.