It could be anything, really. I hate the doctors who throw antibiotics at everything. If it's not a bacterial infection, you don't need antibiotics! Likely it's just viral and will run its course. If he starts having pain, fevers, or BLOOD, get him to a doc. My Crohn's Disease started out as "just IBS" and then progressively got worse and worse. I oftentimes have to wonder if I'm really flaring or if I've picked up a virus. Anyway, try to not borrow too much trouble for another few days. If it doesn't let up or it gets worse, definitely get him back in to be seen, preferably by a specialist.