According to the Language of Scouting, “Webelos” is not a noun. The term to use to refer to boys in a Webelos den is “Webelos Scouts” not “Webelos”.
Here is the LOS entry for “Webelos Scout”:
“(Pronounced WEE-buh-los.) A Cub Scout who has completed the third grade or is age 10 but has not yet completed fifth grade or reached age 11 1/2, and is a member of a Webelos den. The word “Webelos” (means WE’ll BE LOyal Scouts) should always be used as a modifier, never as a noun.” [Bold added]
Incidentally, “Webelo” is not a word.