Depending on what happens in your opening scenes, you may be able to refer to him as "the boy" or "the unnamed person" or some such.
But: How do you suppose that the people in this culture refer to those who don't yet have names? If you're in the person's presence, you could point and say "him", but what if you're not? The fact that you are struggling with this for your narrative may indicate that you have created a society which, to this extent, is unbelievable.
I think this is a common trap for people writing fantasy and science fiction. It's easy to write, "The people in this society all walk on their hands." But really, it's so wildly impractical, how long would such a thing last? Surely sooner or later someone would say, "Hey, it's so much easier to walk on your feet!" and no matter how much the authorities fought it, the advantages would be too obvious. At the least you have to explain away the obvious problems.
Perhaps in this culture unnamed people really do have names, they just don't consider them "real names". The name he's given as a child isn't considered a name, it's just something you call him by until he earns a real name. Depending on what you're trying to do in the story, it might work to say, "He was called Flinghar, but that wasn't his name. He hadn't yet earned a name. He hoped to earn a name in the Trial at the next solstice ..."
Or maybe they're referred to by some description, like "Hamgar's Second Son" or "Boy of the Third New Moon" or some such.
If the society is highly ritualized, I'd think there'd be a formula to such things. You wouldn't just use a random nickname.