That thing on his knee is probably a calcium deposit, a cyst or a lipoma.
I could be mistaken, but I think lymph nodes are located behind the knee (popliteal fossa), much like their being in our armpits (the axilla) and in the pit (cubital fossa) of our elbows. We have somewhere around 800 lymph nodes. They are found in clusters all over our anatomy. The is a multiple of reasons the lymph nodes swell. Fortunately, cancer is among the least common of them.