Well, you could check out my blog and look at the sections for the history of steel pens in the US in the 1830's and 1840's to get an idea of what american pens were being made. By the late 1830's, British pens were the most common, with Perry (the Perryan Pen), Gillott and a few others being the most widely sold. There were a few American makers, but they weren't quite as common as the British makers.
Now, I make no claim to being an expert. I have seen a lot of early-to-mid-19th-century documents. If I had to guess I'd say that he was using a very well-cut quill.
My very unscientific analysis is looking at the fineness of lines. It's very hard to get a very thin line on a quill, and most steel pens at the time were made quite fine, and sharp. The lines I see in the letter are a little thicker than the lines I see coming from steel pens at the time.
Also, as the letter commences toward the bottom of the page, the quality of the line just ever so slightly deteriorates. This could be that he didn't shield the bottom of the paper from the oils of his arm and so the ink is less likely to be crisp and clean, or that the very nicely made point at the top of the paper is starting to be not quite so nicely cut.
The main problem with quills is that they don't last that long, especially if they're cut to a point, rather than a very slight stub. The quality of the line begins to deteriorate. Now, just one page seems like a rather quick degradation, so I could be wrong on this, but I think I might be right.
That's my thought. If I could see further into the letter, it may be clearer. But it's a very cool letter, and great penmanship and a lot of fun to see.