Hmm, he is close to 6 months, am thinking. One of my last hatches, he is an OE roo.
No matter the age, whenever a bird is "down in the legs" a Thiamine/Riboflavin deficiency should be one of the first suspects. Vitamin E can also play a role.
It definitely won't hurt to go that route in any case.
Is it possible that he may have jumped from some height and injured himself? I did have a cockerel about that age that seemed to injur himself. There was no apparent evidence of injury that could be seen or felt, but he was one for going up on high places and jumping down. It was just as he was coming into that age that he wanted to mate everything that he saw (at least 3 x in quick succession) . Anyhow, after not being able to see anything external, I did offer him the raw liver...which was kind of a joke because he kept giving it away to the ladies! I even locked him by himself so he'd eat it and he'd feed them through the fence.
I did manage to get some down him...
Anyhow, I just kept an eye on him. He had bright eyes, was healthy in all respects except that he didn't want to put weight on the leg. By all observances he was not experiencing a deficiency as he
exhibited no malaise whatsoever. Eventually he learned to hop on one leg...it was hilarious to watch him because he wanted to protect the flock and he got really fast on one leg - sometimes even faster than the others. Hopped like that for at least 6 weeks. I began to wonder if he'd ever use that leg normally but eventually he started putting more weight on the leg and then at some point was walking like normal.
The other thing that was funny - because he didn't have full strength in his leg, he couldn't keep the ladies "against their will". They learned really fast how to side-step him if they didn't appreciate his "advances". It was probably the best thing for teaching a young fella his manners real quick. But, heck, what torture for a "teenager" to be given the slip when all these "women" were around to drive him crazy! Honestly, I think I enjoyed that part a little more than I should have...but it was quite hilarious.