It's good that he's got an eye open. I would remove the mucous with a qtip wetted with saline or plain water if that's what you have. Then put a little ointment around the eye. With fighting they can sometimes get cut or dirt in there and the eye can get infected. So keeping it cleaned out will help.
To me, Egg is always good to give a chicken. I like to give mine mashed up scrambled eggs. As for poultry feed - I would use Chick starter or an all flock feed. These usually have 18-20% protein. I prefer 20% for my roosters, but 18% is good - just depends on what you can find! Sometimes they want something "unusual" when not well - tomato, melon, little tidbits of this and that - they don't eat a great deal of one thing. You can try different scraps and bits to see what may interest him, but if he will eat egg that's a good thing, imho.
Vitamin E helps with neurological symptoms - seizure like symptoms, motor skills, balance, etc.
B Vitamins are helpful - B2 for legs, B1 assists E with neurological symptoms, some of the other B vitamins can help with appetite and overall health.
Most human B-Complex tablets also contain C. Poultry Vitamins like Poultry Cell (which is what I try to keep on hand) have a wide range of vitamins/minerals.
I do keep human vitamins on hand too, so switch up, but only use them if I have an ailing bird since their feed should supply what they need for the most part.
Hard to know about the legs at this point. Could be nerve damage, fighting is exhausting, but getting him to move a little is good. Hopefully nothing is broken and he's just very sore and bruised.
You can try making a sling for him to see if that helps. It allows the bird to be upright but they don't have to bear their full weight. Some birds will not tolerate being in a sling, so you would want to monitor him closely. If he's thrashing about, then he may flip out of the sling further injuring himself too.
If he can hold his head up, a sling might be helpful. If not, then rolls of towels to help prop him up may be better.
There's a lot of images of chicken slings if you google them or search here on BYC, but this short video gives you a concept of what one can make.