What causes a leopard gecko to get an abscess and is there anything you can do to help him get well?

Abscesses are common in reptiles. In almost all cases, it is best to drain the abscess unless doing so will cause greater damage. Once the infection has been cleaned out, put some betadine on it. From there there are two options. As crazy as it sounds, packing the wound with calcium carbonate (if it is somewhere external or not in the mouth) does a great job of preventing infection. If in the mouth, swabbing it with listerine (50% dilution) twice a day for a week or so will often to the trick. If more severe, or if you would prefer, 0.66 mg of Baytril per 10 grams of body weight every other day for 10 days will almost certainly prevent infection from becoming reestablished.