I think that's a very broad generalisation
- and I'd expect as much differential between different lines within the UK, as between those and US dogs. Most UK boxers, like most boxers in other coutries, finish skeletal growth in the 14-18 month range (which is already quite late, for a dog of this size). Full maturity may take rather a lot longer, but that's something of a different matter.
Well, you can leave him till 2 years, of course. It is only another six months. But it would surprise me if one side of his jaw only were to make a sudden growth spurt. And is still worth bearing in mind that a wry jaw is a structural deformity that may be passed to offspring in
greater proportion than the parent is affected. Slight or not, it is not a minor cosmetic fault you're talking about.