Fishkeeping - Green FrogTadpole?

I have a tadpole that i recently purchased at a pet shore here in IL.
They don't know what kind of tadpole it is, however they thought it
was a Bullfrog or a Leopard.

It's very olive colored, with a yellow belly. Body is smothly round,
and tail is longer and smooth (the profile from the side is not waivy
like some species). It's about 1 3/4 - 2" in length. There are not
really any spotting on the tail or body. It really looks like an olive
with a tail.

Now, I have seen bullfrogs before, and it seems a bit small, and lacks
spotting that I remember for a bullfrog; I think the same for Leopard,
too.

Now, here's the kicker: They said "we don't know why, but some develop
right away into frogs, but others will not develop until after winter,
in the spring..." This is leading me to belive it's a green frog.

Right now, I have him in a 55gal with a few fancy comets. He seems
very happy. I would like to keep him there as long as possible.

Any thougths as to what he is? I need to know if I am going to have a
frog this fall, or if I need to plan next spring.

Thanks to all,
Jeffery