You are kidding right? You really think that Boomie is a proper Hindu/Indian name?I'm curious. You must have a screen shot showing where it was written - on screen - that the name is spelled "Bumi," yes? Otherwise, you're pulling it out of your butt, and ASSuming you're right.
Well, it was spelled "Bumi" in the original show, and Aang clearly named his son after that character, so it stands to reason it would be spelled the same.
Was there a note, or a book, or banner, or something else, that was depicted on screen showing that spelling? I'm able to find five episode titles that have character names in them, but "Bumi" is not among them.
Jet, Zuko Alone, Appa's Lost Days, Sokka's Master, and Sozin's Coment Part 4: Avatar Aang.
http://avatar.wikia.com/wiki/Bumi