In my experience, at his age, this may well be Marek's disease which is a herpes type virus that can cause paralysis of a leg or wing and sometimes neck. . Have you added any new birds to your flock recently? It would take several weeks to show up and is usually triggered by a stress factor.... perhaps adolescent hormones in his case. Is he still bright eyes and eating well? My experience is to offer support and good nutrition and they will either get over it or deteriorate. If they recover, they will be a carrier for life and may have subsequent more serious attacks, weeks of months later. Unfortunately if it is Marek's, there is no accepted treatment, but supporting the immune system and keeping them happy and stress free will help. The rest of your flock will almost certainly be exposed to it by now, so keep an eye an any other youngsters
Hopefully it is not Marek's and he just has something simple like a strain, but in my experience strains are rare and Marek's is common. It's certainly worth reading up on it, so that you are prepared if that's what it is.
Good luck with him
Barbara