I bought DS these chocolate stirrers here - balls or square pieces of chocolate on a stick that you stir and subsequently melt into a mug of hot milk/chocolate.
here are different flavoured stirrers from Whittards.
Or buy a mould to make your own and get creative with flavourings, types of chocolate-
www.amazon.co.uk/Chocolate-Biscuit-Dippers-Stirrers-Silicone/dp/B0092DILA2/ref=sr_1_6?ie=UTF8&qid=1353091090&sr=8-6
Browse here at Whittards and get different hot chocolate flavours although for a treat I'd suggest Charbonnel & Walker Chocolate Flakes which are so delicious. Yes, they cost £7-8 a tin but it lasts and it is real chocolate flakes.
This looks fun too.
www.whittard.co.uk/christmas_gift_shop/christmas_gift_sets/chocolate_biscuit_ball_sachet.htm
I would also look for a proper French 'Chocolat Chaud' bowl, the wide shallow design that people have their Breakfast coffee/chocolate out of. They just look so stylish and make the act of drinking it more pleasurable. Their wide shape allows you to wrap your hands around it which ius even more comforting.
These Dibor drinking mugs are a little pricey but I have posted the link to give you an idea of what I think looks really stylish- John Lewis, HoF, M&S etc all have them.
www.dibor.co.uk/Product.asp?ProductId=Q102
Finally, you can make shapes and designs on top of the hot chocolate using cocoa powder/grated chocolate in the same way Barista's do by buying the little discs with cut out patterns on them. Or make your own by tracing designs onto stiff acetate then cutting them out. Your son might enjoy designing his own too as Smiley faces, cartoon character heads, logo's, hearts, bug and other animal designs are not too difficult to do. I know that last Christmas, Boots were selling little packs of these in their Christmas gift range.