If your son is on crutches still, he will need to use the elevator to get into the terminal, rather than the escalator. It is to the right of the valet parking kiosk. You should consider wheelchair assist, as knee surgery can be a b*****. (From a mom who has had to deal with a son with knee surgery.) I think that wheelchair assist starts at the elevator. They will take you through check-in and all the way to the atrium.
You might want to consider renting a wheelchair, as crutches get very tiresome after awhile. They aren't very expensive and that way you will have one if he should need it. Handicap muster is in the main theater. If he is in a wheelchair, they will let you in a little early. The theater is also used as muster station A and is packed like cattle, from the front row to the back, as close as you can sit. He will not have to walk up the stairs to get to the lifeboat if he is not able. If you have "registered" with Handicap Services, his name will be on their list as needing assistance. If you have not registered, it is no problem to stay in the theater after all the others leave to go the lifeboats. They can add him to the list then.
Not knowing his age, I know that my 19 yo would never have been able to make it on crutches 2 weeks after his MPFL. He was all tough and thought it was going to be a piece of cake. Turns out one of us had to help him get onto/off the bed/couch as he could not use his thigh muscles to lift or lower his leg. Took him about two weeks to be able to get around more easily, but it was still painful. So....don't forget his pain meds just in case.
Good luck and enjoy your vacation!