That's going to depend on what you mean by "separated." If you're simply not living together and don't consider yourselves married, that's all well and good, but the law doesn't actually recognize such a state. You're married. You aren't under any legal obligation to have your spouse on your employer's health plan--you wouldn't even if you were married--but you might find yourself liable for his heath care expenses anyway.
If, however, someone has filed for divorce but the divorce is still pending, that's a different matter. A lot of the time, joint assets and debts stop being counted once the divorce is filed.
It sounds to me like you need to consult with a divorce attorney before you make any decisions here.
This answer does not constitute legal advice and does not establish an attorney-client relationship.
Sign up to receive a 10-part series of useful information and legal advice about the divorce process.