Weigh however many eggs you want to use, in their shells, then use that measurement to weigh out the same amount of butter, sugar and flour.
Then beat the butter and sugar together with a wooden spoon. Mix your eggs in a jug and then start to mix in the egg a bit at a time until it's all incorporated. Add a bit of flour if it starts to curdle. Then sieve in your flour in a couple of goes, and mix it all up. If you have any cocoa powder, vanilla extract etc this might be nicer, otherwise it's just a basic sponge.
Bake it at about 180c for about 30 min or until a cake skewer comes out clean. You could split it and sandwich together with buttercream, jam or chocolate icing. (recipe pretty much nicked somewhere off mumsnet, but it's a steadfast for me!)
I'm sure he'll love it, chocolate and smarties are always good! If you have lots of smarties you could even smash some up and bung it in the mix for a different texture and a splash of colour. Hope it turns out ok for you!