I think when it says you get a bonus on your damage roll equal to twice the number of sneak attack damage dice you roll it means you would add +2 to damage not +2d6. I could be wrong though.
One of the requirements of becoming an Arcane Archer is being either elf or half-elf. Were you ignoring this and letting him take levels in it anyway or did you forget about that?
Try a sorcerer with the fey bloodline. Or you could have the wildblooded version of it. At 15th level you basically become a fairy. Either way you should check out Gnomes of Golarion so you can really get a feel for Gnomes.