My Son Cut His Hand Last Night, With A Knife.

My son cut his hand last night, with a knife. Two doctors have looked at it. They both say the tendon has been cut through partially. He has stitches and is supposed to see a hand surgeon on Friday. Up until a couple of hours ago, he said he wasn't feeling much pain. Now he is starting to feel pain, but not in the same area as the cut and his hand is swelling What could be causing the pain and swelling now?