What Can Cause Problem In Bending Knuckle Right Behind Thumb Nail?

My son has a basketball injury from 4 days ago, Ball hit his thumb and he had a bit of swelling a alot of pain , also not being able to bend the knuckle right under his thumb nail, took him to the er that night, doctor took an xray and said nothing is broken and had no idea why he could not bend that knuckle, blanmmed it on him being scared to bend it, well 4 days later and he still is not able to bend it?, any ideas?