Aw, this post kinda makes me sad. I hope that when my son gets older I have an open mind about things and lets him do what interests him, not just what I think is worthwhile or "safe."
I'll second what noodle said: You may gain more traction by explaining that a college scholarship is quite possible with a solid fencing background, and present him with a
list of NCAA schools with varsity fencing teams.
Could he be concerned about the safety? It's a fact that fencing is safer than sports like soccer and football, because there aren't as many contact injuries.
Is there some other reason he could be objecting? Did he have a bad experience with your brother's coach or club? Perhaps your bro could shed some light.
Does he want you to settle in some activity and pursue excellence there instead of losing progress by activity-hopping?
If you can't make any headway, take comfort in the fact that once you leave for college he won't have much say in the matter (except, perhaps, monetarily). Just make sure you go to a school near a good club!