Son's Knee Swelling nearly a year after Patella Surgery

[DShahan]

My son had patella surgery April 2015 for a reoccurring stability issue. He hurt it playing baseball.  He was out 12 weeks and rehabbed and returned to sports by July. Shortly after returning to playing he had an issue where his knee swelled and had fluid drained and a cortisone shot.  Since that time he has continued to condition over the winter for this Spring's baseball season and had no other problems and the knee appears sound. Recently he dove for a ball and his knee swelled up over the weekend.  From what I can gather it sounds like a contusion to the same knee.  Is this type of swelling unusual and to be expected? He doesn't where any type of brace at the advise of the surgeon. What advise can you provide?