My son has multiple food allergies, including milk, and I've tried different mixes each birthday. The better ones I've used are cake mixes from 123glutenfree.com (shipping is expensive, but they items taste very good) or Gluten Free Pantry brand. I found this at Woodman's in North Aurora. Whole Foods probably has it also. There is also a Cherrybrook Kitchen brand that I've seen even at Wal-Mart and Jewel, etc. I haven't actually tried those because it has gluten, and he's allergic to that, too, but I've heard that brand is very good. Check the icing brands in the store because some are okay for food allergies. I use the Funfetti kind b/c it only has soy and that's the only thing he's NOT allergic to, but since that's not an option for you, just check the labels because there is probably one you could use.
When my son was very young, he didn't really know the difference so I just had a plain cake for him or cupcakes and a regular cake for everyone else decorated with the theme. This year, he just turned 4 and can tell the difference so I used the Gluten Free Pantry brand and had it decorated just like his other one (Wall-E) and it turned out great! Wall-Mart let me buy the WallE cake toppers w/o buying the cake for five bucks.
Happy Birthday to him!