This is exactly what D had. It was the result of a soccer injury. This is very common injury among female soccer and basketball players.
Her knee cap used to randomly dislocate with no warning and it became dangerous for her. She fell down a 6 or 7 outdoor concrete stairs at school because her kneecap dislocated as she was walking down steps. Another time her knee locked and she fell off a hiking trail and slid about 6-10 ft down a slope.
Your son will probably have 2 or more screws placed in his tibia right below his knee to anchor the bottom of the tendon the surgeon will use to replace the damaged knee ligament. The screws have to stay in for 1 year. He will be able to feel them when he kneels. After a year he will have the option to have the screws removed, although most people just don’t bother to.
He’ll also a set of parallel scars about 6 inches long on both sides of knee. Makes for great “war stories” at the ski lodge.
And D was driving an automatic transmission car about 3 weeks after her surgery since it was her left knee. She got a handicapped parking placard and it was s wonderful help since using crutches to get around is PITA.
If your son’s surgery is on the left side and he has a car he’ll be driving, tell him to ask his surgeon for a letter of need for the DMV to get temporary handicapped parking permit.