Impossible cake! Can you help?

May I suggest a test before the party?  My mother had this recipe for a chocolate slice that had an oatmeal crust and a cocoa-egg-sugar filling.  The good thing was that you could leave out most of the sugar.

I wonder if this can be to your son's taste for fudgy and definitely not too sweet.  If so then you can make a layer cake with 3 layers of this goodie baked in cake pans, and stick it together with just a little bittersweet ganache and not ice it at all...maybe put a printed image on the top to celebrate his birthday.

Let me know and I will fish that recipe out. 

How big does it have to be?   You can take a bunch of the largest tootsie rolls you can find and microwave them, just a few seconds each, until they are a little pliable.  Once soft enough, you can use them like modeling chocolate.  Combine them together and roll one really large tootsie roll or shape it in any mold you have.  You can do the same with the colored flavored tootsie rolls for decoration (we make roses out of tootsie rolls), really it can be shaped and molded just like candy clay/modeling chocolate and fondant.  You can do the same with starburst candy.  Remember just a short time in the microwave to soften it up (careful not to melt it!)

Most kids love cake. Maybe you have not found the right cake that he would love.  I am very surprised to hear that a kid does not like sweet things.  That is very unusual!!! Hopefully someone here will have some kind of idea as to what you can make. I have no idea. Good luck/Mary