I have tried experimenting with mixing nutella and icing sugar and DC thought it amazing as it was so sweet, but it was a congealed mass to spread and I thought too sweet. Nowadays I melt a big bar of milk chocolate in a glass bowl over boiling water, and when it is soft I add some icing sugar and a tiny amount of butter, and that is the best I have made, it tastes amazing and almost fudge-like when it cooled.
Cadburys used to do a ready made icing, used to get it in Sainsburys. I haven't seen that for years though.
Does your son want a chocolate cake? If you did decide to go for that and they are older then 12 I have an amazing US recipe which DC asks for all the time - not v healthy - 2 cups (medium size) self r flour to 2 cups of white sugar, 1/2 cup cocoa, 3/4 cup filter coffee, 3/4 cup mayonnaise. While using an electric mixer to mix, if it seems a bit stodgy use a splash of milk. Once mixed, taste and add more of less cocoa and coffee and mayo as desire. It is absolutely amazing with the melted choc icing. Bake for around 35 mins, but depends on oven, keep an eye on it.
When DC under 12 I avoided because of the coffee and mayo, but others may be more gungho!
The melted choc icing would go nicely in the middle of plain sponge too, and I usually melt more choc and do a pattern with that over the top with some icing sugar sifted over the top.
(not relevant to thread but we are having a cake summer, next on the list is cinnamon flavoured sponge and icing)