Probably an infection or abscess. Check the sole of his foot to look for cuts or other injuries. Also check the base of the claw. Possibly he snagged his claw on the cage screen and partially pulled it out. It happens. If there is no open wound in the skin a topical antibiotic won't reach the infection, so you may need a vet to open up the abscess, clean it out, and put him on a systemic antibiotic. Herp abscesses don't drain as easily as mammals' do. The material inside isn't liquid, its more like cheese. If you happen to end up with Baytril (a very commonly used broad spectrum antibiotic) be sure you hydrate him VERY well. Its hard on the kidneys.