As usual, I have to agree with Ms. Brewer. The best course of action would be for your father to execute a codicil with amends his well. If that is not feasible the first brother should decline to qualify as executor when your father passes, but he will have to be agreeable to proceeding in that regard. If he is not agreeable then you would have to go to court and prove his is not fit to serve, and as you've already heard, that can be a very unpleasant process.
I strongly encourage you to call a local estate planning attorney (or perhaps the same attorney who originally drafted your father's documents) and have them prepare a codicil as soon as possible.
NOTE: If you find this response helpful, please click on the “thumbs up” button at the bottom.
DISCLAIMER: THE INFORMATION PRESENTED HERE IS GENERAL IN NATURE AND IS NOT INTENDED, NOR SHOULD IT BE CONSTRUED, AS LEGAL ADVICE. THIS POSTING DOES NOT CREATE ANY ATTORNEY-CLIENT RELATIONSHIP BETWEEN US. FOR SPECIFIC ADVICE ABOUT YOUR PARTICULAR SITUATION, CONSULT A QUALIFIED ATTORNEY.
It would be best if your father can muster the energy to revise his will. If your father passes away, it will be necessary to show the probate court that you've made efforts to locate your first brother. This could cause delays. Also, if you find that brother, you would have to prove to the court that he would not be an appropriate choice for executor (unless he declines to serve). That could be a somewhat unpleasant proceeding.
The information you obtain at this site is not, nor is it intended to be, legal advice. You should consult an attorney for individual advice regarding your own situation.
You have already received some good advice. If your father is living and able to make decisions regarding the executor on his will, then the only thing that you can do during his life is to have conversations about the situation and recommend that he change the list of executors. After your father passes away you will be able to contest the nomination of your brother as the executor in the probate court, but unless you have a good reason (and plan on making it part of the public record in the hearings to appoint the personal representative), the court will generally respect the nomination of the personal representative in the decedent's will.
Alternatively, you may be able to file a probate petition stating that your brother's whereabouts are unknown and that you or the successor executor should be named as the active executor by the court. The court will ask you about the actions taken to locate your brother and if he is unresponsive or will not appear then you may have a chance at having the court appoint someone else besides him. If he does appear though you will have to go through a hearing and the court will decide who shall act for the estate.
The easiest thing for you to do is to continue to have conversations (even though they will be difficult) with your father. If he does not change the list of executors and passes away, then either way it will probably delay the settlement of the estate.
If the estate planning attorney who drafted the will is still the attorney for your father, then recommend that he have a conversation with that attorney about the qualities that an executor should have, and about the problems that your brother is going through. If he/she is a good estate planning attorney then he/she should recommend that your father change the list. Good luck.