You need to get him up and get circulation into his legs. Make a small cradle for him out of an old tshirt, tie it so that when he is stood up it will rest under his belly and take most of his weight off his legs. Put him in the cradle 4 or 5 times a day. Pen & strep for 3 days, if no better give him another 3 day course. Had a lamb like that last year and this seemed to do the trick.