Chat/email/voice are more than sufficient to establish a contract in the US legal system. Read the introductory "elements of a contract" section from here: http://www.expertlaw.com/library/business/contract_law.html
What you can do and what you should do are different things. If he's not keeping up his end of the contract, then sue him in the country the company is registered in. Don't give him time or the benefit of jurisdiction in any legal action. If you just don't like the guy, he likely still owns part of the business.
IANAL, but chat or email probably count as "in writing."