Baker's Cyst. He probably has a baker's cyst of the knee. This is an outpouching of the capsule of the knee. More sever problems could be a clot, an aneurysm, a tumor or just some localized swelling and edma fluid. Rest and observation usually take care of this. Otherwise, see pcp or orthopedist and get an evaluation.
Baker's cyst. It's probably a baker's cyst, which is the result of either arthritis or a meniscal tear. These entities can produce fluid in the joint that pouches out in the back of the knee. See a sports medicine knee specialist for treatment options.