Honestly if he is happy keeping the build I wouldn't worry about it. You have plenty of melee and damage. he is gonna be overshadowed without a HUGE push in a specific direction. I would just give him an interesting item familiar or such. Maybe it has very little effect in combat but it has a very memorable personality. Maybe it hates the fighter but refuses to be held by anyone but him. Likes the color purple and refuses to attack anything wearing purple. Maybe it always talks out loud at the most inappropriate times, or better tries to engage the fighter in philosophical debates in the heat of battle. Perhaps it refuses to be unsheathed unless the Player says "I HAVE THE POWER!!". Randomly takes control of the character and makes him do odd things in social situations.

Any of these things would do more for a first time player than giving him bigger damage numbers ever will. Plus since the other players are more experienced and more prone to minmaxing you can easily keep them from initially trying to grab this item.