That is very sad. Here is a basic first aid article by another member, Paul Rust- http://www.frogforum.net/showthread.php?t=2743
If you think he will be able to care for himself once he heals I would let him go when he is better. If not I would keep him. I suggest giving him a honey bath (fill a small dish with lukewarm water no deeper than the frog's chin and dissolve about a teaspoon of honey then place the frog in for 15 minutes.) Honey has antifungal and antibacterial properties and the sugars will give him a bit of an energy boost I would also switch to nightcrawlers as a staple instead of mealworms since they don't have much nutritional value.