If you don't want it to die, you need to get a positive ID on it and spend the money to make a habitat it can survive in. You also need to make sure you have a ready food source for it at this size. Unfortunately, without an ID, you have no idea what kind of habitat it needs. I think Brian was correct on the narrow mouth toad guess. If so, it's ALWAYS going to take a small food source. So you will need to learn how to culture fruit flies or start ordering them, or have a way to get small crickets regularly. It only will grow to be about 2 inches at the most, more likely settling in to 1-1 1/2 inches. You will need a tank, with the correct substrate, coconut husk fiber usually works well. This species is nocturnal and will burrow during the day. You will need to have a water dish that he can get in and out of easily, and make sure it's not so deep that he will drown, enough to cover the belly is fine. You need a way to maintain temps in about the 70's and need to keep the humidity up, although I'm not sure of the exact level... Just doing a quick search on these shows that they don't make very good captive pets. If you start googling and reading, you'll find a lot more info about them.

Remember, just because he appears to be doing well now, doesn't mean he'll continue that way without the proper habitat and diet.