Looks like his tendon may be slipping a bit.thought it could be a double jointed but also now thinking if I would have noticed it sooner if he was. He is around 13 weeks old and very kind. He doesn't seem to have issues with walking around or jumping off the roosting bars.
You can try stabilizing the leg and joint by wrapping it, but it would need to be checked frequently to make sure the wrap is not too tight and that the skin is looking good.
I'd also offer supportive care by giving him B-Complex that contain B2(Riboflavin). For a growing cockerel that is leggy, I would give 1/2 tablet daily.
What's the protein % in your grower starter? At least 18? If so, that's good.