Bruising After A Break

My 12 year old son started to complain about knee pain. So I took him to the doctor and X-Ray's showed a fracture on the knee cap. He is in a knee brace that prevents him from bending the knee. He also going for a MRI on Saturday.
He has been in the brace since Tuesday. Today his knee is starting to bruise and swell. He is still being active and I can't understand why this is happening now. His brace isn't too small and everything about his knee is padded.
We have no idea when or how he broke his knee cap. I think it happened in Early May when he fell. He complained for an hour afterwards that his knee hurt but nothing again until a week ago. He hasn't fell or injured himself lately. He has been walking normal, running around, biking and swimming. But he suffered in silence and would only tell us about his pain when he couldn't stand it any longer.
So why would his knee start to swell and bruise weeks after the knee was broke?

[DrNavneetMD]

Hello Dear
After a knee cap fracture there is intense knee pain. Almost the knee becomes swollen and the patient is unlikely to be able to bend the affected knee. If the Patella fracture is right across the knee cap it may be possible to feel a gap.
Do not worry it is a normal phenomenon.
http://www.physioroom.com/injuries/knee/patella_fracture_full.php
Best