Hopefully someone with more experience can chime in here, but what kind of deal do you have with him (i.e. can you fire him)? Either way, I'd consult with a lawyer to see what your options are
We would much rather continue working on our own without him.
Fire the CTO and ban him from the premises. You and your investor control 95% of the company, you can more than easily ensure he is a total non-entity to your company forever.
Talk to a lawyer