Sick mouse, help!

I have a mouse about 4 yrs old. Found him in the barn when he was first born. Noticed a knot on the inside of his back paw. It's about the size of a dime. He looks sick in his eyes. Have rescued and cared for lots of different animals, and raised Mr. Jack this long with no problems, but I don't know what to do about this and not sure what it is. Any information would be greatly appreciated.