I have finished his "cake". Its a little messier looking than I hoped it would be as I struggled with the joining seam but here it is.
Its a chocolate smash cake. Just a chocolate shell filled with a variety of sweeties. He needs to smash the shell and out come the sweets.
It was easy enough to make (although a little messy) as I bought a giant cupcake silicon mould but I did find it difficult joining the two shells together as they didn't match up exactly. But it tastes nice (I used proper chocolate rather than cooking chocolate) - I have been nibbling the bits of chocolate I shaved off the edges.