Is your son really that picky? Ask some of the natives, they should know some good places.
Hopefully you will be able to bake the cakes for your son yourself soon, but in the meantime use a reliable bakery for these situations. A favorite of mine is edelweissbakeryfl, since I've never received anything that wasn't tasty from them. From bread to cakes: they can do it all and you will wish you ordered more when you're done eating.