I didn't realize that he had gone incommunicado. Regardless, I would still try to reach him from any possible form of communication and talk to him.

If he is clearly avoiding you guys while continuing to misspend company money, then I would talk to a lawyer. Did you guys set up any kind of vesting schedule for each of your 20% of the company? It's a fact of startup life that founders will sometimes leave, and vesting schedules help ease the legal mess that ensues.