Should I use concrete for a 2" square metal fence post?
Eight months later, here is my answer and update: 

I called the my town's building division and determined the frost line for my region is 48". It wasn't possible for me to dig below 48", so I dug to the frost line and instead of using cement, I filled the hole with pea gravel (roughly three 0.5 cu ft bags), tamped it down with a 2x4, and topped with some of the soil I had removed when digging. My thought is the pea gravel will allow draingage, while still providing enough support for my narrow fence post. This might not work for larger posts. 

So far, the fence post has been very sturdy and having gone through one winter, has not risen. I will report back if I find any issues with my solution, but so far, I'm satisfied with it.