Is Aaron a cofounder of Reddit.com?

Yes.

This is easy to see from the historical record. For example, when Reddit was acquired by Condé Nast, they and Reddit jointly issued a press release which called Aaron a founder:

The deal was announced today by Kourosh Karimkhany, General Manager of Wired Digital; the four founders of Reddit: Steve Huffman, Alexis Ohanian, Aaron Swartz, and Chris Slowe; and Paul Graham, founder of Y Combinator, the venture firm that provided funding for Reddit.

Paul Graham, Reddit's major investor, also confirmed Aaron was a founder:

Aaron's not wrong to call himself one of the founders. The company behind Reddit was a merger of two startups ... and in that situation the founders of both startups are considered founders of the combined company.

As far as I know, there's absolutely no historical evidence to the contrary.

changed November 2, 2012