If it were me I would make your usual cake, cook it in 1 large & 1 small deep pudding type bowls
then pile one on top of the other, sticking together with some sort of jam etc that he can eat
cover with a bit of your chosen jam - cover with rolled white icing - you can buy it ready made to roll out, or a ready rolled sheet.
cut out discs in icing too
mix up some sugar paints, by adding a drop of food with icing sugar & mix to a paste, thick enough to stay put, thin enough to paint with
use clay tools, knives, thin rolled out tubes of icing etc to make the rest of the detail & then paint with the sugar paint as you would anything else
When you say he's allergic to blue food colouring - do you mean the artificial stuff??
if so, my own DD is allergic to all the artificial ones, but its easy to get natural colours now, so blue in those will likely be okay