He can't go into battles as a front line fighter. The dex, con, 6 sided hit dice and lack of heavy armor will get him killed. He needs to be a back line rogue that uses tactics to be helpful in combat.
I'd have him focus on a polearm or other reach weapon. A weapon where he can get others to stand between him and danger, yet a weapon he can still use for melee attacks. Sneak attacks are easier than you think when wielding a polearm. It will cost him a feat, but it will be a must if he wants to do anything useful in combat without dying. If I were him, instead of buying the proficiency with a feat, I'd do it by gaining a level in fighter or barbarian.
His best use to the party will be as a negotiator, if the campaign features a lot of non-combat activiites. Give him lots of skill points in interaction skills: bluff, diplomacy, gather info, intimidate, sense motive, etc ... Some secondary social skills (appraise, knowledge, etc ...) which are useful in discussions with others might be useful as well.