I can't remember if it was on here, but I've heard him described as a "proto-socialist" and I'm wondering how valid that statement is.
No validity whatsoever. In terms of economics, Jeffersonianism is largely thought of as existing in opposition to Hamiltonianism. Hamiltonianism advocated ready access to capital, and envisioned an urbanized, industrialized nation, much like the then forming industrial revolution in the USA. By contrast, jeffersonianism is constructed with the vision of yeoman farmers and other self-sufficient types or self-employed craftsmen forming the basis of the US economy. Jeffersonianism of course also had more tolerance and room for slavery than Hamilton's view of the world and also limited citizenship strictly to white males with property. Both of these visions aren't really redolent of socialism--unless you want to make a 'great leap' and see Jefferson's agrarian vision as being proto-maoist--but Hamiltonianism is far more recognizably similar to our own economic world and also had a larger place for centralized government power.