I imagine this will get ugly if you just change the passwords and revoke his access. He owns a decent share of the company so my suggestion would be if you wan

I imagine this will get ugly if you just change the passwords and revoke his access. He owns a decent share of the company so my suggestion would be if you want this company. Buy his share and give him the money. During this period of time you can hire a team (or someone) to secure your companies credentials. Hiring a team to review your assets both physical and virtual; code base and otherwise would be wise.