I'm sorry. It sounds like paralysis.
Some forms can be cured, others can't. First, try giving him multi-vitamins meant for chickens, including riboflavin. If you don't have these on hand, get some scrambled eggs into him fast. Keep him warm and hydrated. If the legs seem weak, you could try splinting them.
My chick was 4 weeks old when it suddenly developed the same symptoms. It never healed, and we had to put her down. It was really sad. All I could think was that it had to be marek's or else something genetic. None of my other chickens are showing symptoms.
I hope your chick gets better soon!