Are his funds considered lawful if I enter into a financial partnership with him, with him providing the capital and me the effort, on the condition that he writes over to me a portion of the company's shares, knowing that he accumulated his wealth from excavating and selling antiquities?

The ruling on buried artifacts in the earth varies depending on their type and condition. It is not possible to rule on the permissibility or impermissibility of this man's money without knowing its source.

If this money is permissible, and the questioner wishes to engage with its owner under the Mudarabah (profit-sharing) system, then it is not valid for the owner of the money to have shares. This is because the worker's compensation in Mudarabah is a common share of the profit, if any exists, and it is not permissible for him to have a fixed wage.