How to fire a co-founder

Most probably, your cofounder thinks that he has already earned his equity till next March and therefore taking the time off. You should confront him and give him your feedback both verbally and in writing. Because you have a buffer time of 1 year, you should provide him enough opportunity to fix the situation. If after your repeated communication, he does not take corrective actions, then you can formally fire him. You may also want to consider providing him some equity, sufficient to cover his initial hard work. In return you should get a declaration signed off wherein he agrees to forgo any past or future claims on the startup equity, other than what you offer him at the time of termination.

He is a co owner with a 20% stake. Buy him out and next time spell out what is expected of equity owners. Otherwise, if he does not have a serious misdemeanor or felony not much way. Look at state statutes to see how liberal your state is. He is not an employee at will so don't go down that road. Do you have HR policies in effect? If not, then the warning and firing is difficult. Buy him out.