He seems to be experiencing leg weakness. It can be from hypothermia, starvation, a vitamin deficiency, or he could be becoming symptomatic with an avian virus. The symptoms match all those.
Try treating for the most simple and probable first. Give him a cup of warm water with a teaspoon of sugar mixed in with a pinch of salt and baking soda. You can quadruple the recipe and let all of the flock drink it as it won't harm them. If this is hypothermia or starvation, this will improve the symptoms in just hours.
The next easiest to treat is vitamin deficiency. A vitamin B-2 and 12 deficiency will produce leg weakness and I would toss in vitamin E and a sliver of selenium in case there's an E deficiency causing neurological symptoms. Give the B in a B-complex to cover all the B vitamins. You get them and the E any place they sell people vitamins. Give a B-tablet and E capsule directly into the beak. He won't choke. Symptoms should start to improve in around two to four weeks.
If his symptoms worsen after these treatments, then you may be looking at an avian virus such as Marek's or lymphoid leucosis.
