Well, personally, I would wait for a night like Yule, because I'd find the... the flavour of the evening to be more suitable. It also gives you a lot more time to decide what you want to do, what you need, to gather everything together, etc. But that's totally a personal choice.
As for the ritual... well...
*Windwalker waxes poetic and composes a ritual of sorts*
I'd have everyone dressed in a white robe, seated about the fire. In this part of the ritual, I would have everyone older than the youth in question, because they're welcoming him into their number. The youth himself would take a ritual bath with specially chosen oils, dress in a new white robe, and join his elders. Circle would be cast simply at this point. I'd compose some speech at this point, nothing fancy, with a call to Apollo, who was especially important in Greek rites of passage from child to adult. If it was me, I would present my son to the others and make a statement along the lines of "this is my son, a child no more, and I set him before you as a man, and give him to himself", something like when a father gives away the bride. You know what I mean.
There might be a teaching-story told here, a reminder of the responsibilities of adulthood, that sort of thing. Something along the lines of the path that lies before him, of all the possibilities avaliable, of all the different paths he could take in life and in spiritual practice. There would be food, ritual wine (as a man, he'd now be entitled to partake if he wasn't before, at least in ritual). He might choose a symbol of childhood to set aside, give to a younger sibling, something like that. (Remind him to choose something he won't miss too much
) Then there would be a gift: a first athame, or a wand, or a staff of adulthood. A symbol to initiate him among you.
The ritual wouldn't last more than an hour, because people's attentions tend to wander after that long. Circle would be broken, and the youth would lead the way to another predetermined area where everyone could be more relaxed, the children could participate, stuff like that.
You know, I should really write something out properly. Composing random rituals is fun.
Notes: The Greeks would cut their hair short when they passed from childhood to adulthood. Your son might be disinclined to perform such an act, or might already have short hair, but clipping off a little as a symbol might be something else to include.