Looks like Mareks to me. Just because you have a positive Mareks bird doesn't mean others will also show symptoms. Most become immune to it. Personally will see on average one a season that shows signs. Generally between 8 weeks and 5 months. We generally cull as soon as we can tell what it is...
At that age it could be the start of Mareks, especially if it continues to go straight out and he stops walking completely. Otherwise he may have slipped a tendon or has an injury.