Duoing partner to compliment a Warlock?

[Feral]

I'm going to be duoing a lot with my son who got a Gnome Warlock to 31 in the euro beta and is pretty set on rolling a Horde version in retail.
I was originally planning to play a Shammie but I'm not sure if Warlock/Shammie will be a particularly effective combo so any suggestions and reasons (except warrior) would be appreciated

 

[haarewin]

warlock and mage is great combo for killing mob together, all 3 (mage, warlock and pet) attacking the same target is very effective. then if you have more mobs to fight, warlock uses the melee pet and you pick off the mobs 1by1.
if not a mage, i'd go for priest, druid or shaman. all healer-ish things.