Hello My Son Had A Surgery When He Was 5

hello my son had a surgery when he was 5 to pull one of his testicals down because it never dropped when he was born, the urologist pulled it down as far as he could and said it may drop eventually but it never did and he is now 21 and he just went to a dr and asked about it and they told him there is a large chance this could give him cancer, we are very concerned and wondering how bad this could be..please give any advice we need and we will be getting him in to a urologist ASAP anyway..thank you..katie