Check his weapons and other equipment. There is at least one weapon in the game that gives a -100% bonus to experience earned, effectively stopping leveling (I believe it's the Scythe of the Void carried by Ongr the Unbound, the wandering Juggernaut random event). Also check his traits - there are some injuries which carry a penalty to experience gain.

The other thing to do is to check what kinds of enemies you're fighting with him - AI factions will give pitifully low amounts of experience for killing their armies, especially if you have some kind of experience penalty, whereas monsters give you a lot more. Killing a dragon might give you 50 experience, while killing an AI army which could have killed three or four dragons will probably give you no more than 10 experience.