can a co executor remove the other executor from a will?

1) One executor cannot remove a co-executor from a will except potentially by bringing a legal action against his co-executor if he suspects wrongdoing, in which case the court could review the evidence, and if it concurs there is wrongdoing, remove an executor.
Obviously, since there cannot be wrongdoing by an executor until he or she begins carrying out his/her duties, this cannot occur until after the testator (person making the will; your father) passes away.
2) While the testator (your father) is alive, as long as he is mentally competent, he may freely change his will, including changing or removing executors. A will is not set in stone, but may be changed (as long the change is properly signed and witnesses) at any time prior to death.
While you write that your father had a stroke, unless he has been determined to be incomptent now by the courts, the presumption is that he is competent and can make or change a will. If you believe he is incompetent, you likely need to bring a legal action to have him declared such and guardian appointed to manage his affairs--and if he is incompetent, that will simultanesouly stop him from revising his will. If you wish to consider this option, consult with an elder law attorney.
Note that if you don't have the chance to do this (e.g. your father passes too suddenly), you could try to challenge the will after the fact, when it is filed for probate, on the grounds that your father was in fact incompetent when he made it, but if your father has already passed, it may be more difficult to get and present evidence of incomptency then.
3) Your brother cannot use force, threats of violence, or threats of illegal actions (taking property; restricting your father's ability to communicate with others or travel; etc.) to force him to make a change; if he does engage that type of illegal coercion, that could invalidate the new/revised will. But he can nag, "guilt," hector, etc. your father until he makes the changes--as long as the things he is doing are themselves legal (e.g. "emotional" blackmail, as opposed to financial or criminal blackmail), he can try to persuade  your father to do what he wants, as can you.