What Causes Blister On Knuckle Indicate?

My 11 year old son has a sore on his knuckle. It looks like a blister. He squeezed it before he showed it to me. He said it looked like pus would come out, but when he squeezed it nothing came out. Now his finger is swollen and he has a reddish line going from the knuckle to the inside of his finger and then to the top part of his arm, it extends just a little past his wrist. Is this an emergency?