Thanks, great suggestions! I especially like the one on the Dragonfly Saddlery website - it looks really cosy. The great thing is that when he grows out of it, I can use it.
I have one by Target dry that I love. It was relatively inexpensive, and is not wax so is pretty lightweight, which I think gives you more flexibility when riding, plus you can use layers to decide how warm you need to be under it.
I also like that mine has a detachable hood, so I can use it without a hat on too, for days when Im not riding.
Mine came in Navy, purple or green last year I think. As long as you dont go for purple, the colour and style should be boy-suitable.