I would crush the tablet and give it in food, or if he won’t eat, give 1/4 tablet or close to that into his beak to swallow. Will he curl his toes around your finger if you touch his foot? Does the upper leg still feel hard, or is it swollen or green (bruised?) Are you familiar with Mareks disease? Hopefully it is not that, but an injury or vitamin deficiency. Usually that would affect both legs, but the vitamins can still help. A chicken sling chair might be something that you might want to try making, if he will stay in it for periods throughout the day. Let him sleep and rest out of the sling, but it can help if you put food and water in front of him. Here are some examples especially posts 5, 11, 12, 13:
https://www.backyardchickens.com/threads/versions-of-chick-chairs-please.1166308/