The problem is that the name when you look at it rather invites people to jokingly rhyme the two. If those were the names he carried at school in Sheffield it might have been difficult. Of course that might have been the idea, "Boy named Sue" style. "We all know how Sean is pronounced!" - there are a fair number of people who don't, or won't.

Boromir? Well, Sean was the guy who played him in the movie Lord of the Rings.