Ooh, that's nasty. Poor frog. The most likely thing is he could have sliced himself on something in the tank. You can apply some antibiotic ointment like Neosporin (WITHOUT PAINKILLER).

A lfar less likely cause is red-leg. Not eating, lethargy are signs of the fungus (but it's hard to know for sure because it could just be a normal cut that got infected and he's not feeling well enough to eat.) IF it looks like his other leg has noticeable veins, redness, or any sores it's most likely redleg.