Gracie got a brother recently.. hes a cutie, his name is Will

They have been getting along REALLY well and I have caught them snuggling and grooming each other. It is wonderful!
We do have a problem though. I am bringing Will to the vet first thing tomorrow morning to get his foot checked out, but to put my mind at ease I figured I would ask about it here to see if anyone has had a similar issue.
He is not using his back right leg to walk, and whenever he tries to put it on the ground his foot quivers and he picks it right back up. Upon looking at his foot closely, we noticed that one of his nails looks like it is black and blue, is this possible? If so, could this be the source of his pain?
He has also been sleeping CONSTANTLY which is unlike him. He is a kitten, so he normally sleeps, plays, sleeps, plays. Right now he won't even play with his favorite toy. He is eating, but only after I hand fed him some food (he must have tasted it and wanted some more).
I don't even know how this could have happened.. he is an indoor cat that usually hangs around us. I'm really worried.