Without knowing that the problem is with your son’s meniscus, it’s hard to say why one type of surgery would be preferred over another. However, sometimes it is a better idea to open the knee with a traditional incision than do an arthroscopic surgery, with the slender, long reaching instruments.
A child’s joint is quite small, leaving not much room to maneuver. It’s possible that the surgeon feels more comfortable doing the open surgery, allowing him to see the entire joint, than try to do it arthroscopically.