Green is bruising in chickens. He is probably sore or sprained his legs. Make sure that he is getting plenty to drink and eat, since he may starve until he is able to walk. A chicken sling or chair might be an option if he does not start walking soon, but let him out often to test his legs and for him to stretch and sleep. Here are some examples of chick chairs:
https://www.backyardchickens.com/threads/versions-of-chick-chairs-please.1166308/
https://www.backyardchickens.com/threads/versions-of-chick-chairs-please.1166308/
