The Application form says
Webelos Scout—Must have completed third grade but not completed fifth grade, or be age 10 but not yet 11 1/2
So, since he can JOIN the unit, until then, he has AT LEAST until he ACTUALLY finished 5th grade (assume June 15th +/-) or turns 11 1/2, whichever comes LAST. The rule isn't as specifically stated as the "Age 18" rule for Boy Scouts, or the "Age 21" rule for Venturers and Sea Scouts, but that's how I'd interpret it.
Since he can actually be included in the rechartering of the Pack until then, he could, theoretically, remain a Webelos Scout until the NEXT rechartering. So If the Pack recharters in April of his 5th grade year,, as Mrw mentions, he could still be a Webelos Scout until the following April (of his 6th grade year), but I would assume he could no longer earn the AOL during the extended membership period.
The same membership rule actually applies to Boy Scouts, too. Even though they stop earning advancement on their 18th birthday, and must follow Adult YP guidelines at that point, they remain chartered members of the unit and BSA until the end of the chartering year (in Mrw's example, April 30), even if they do not submit an Adult application form and officially register as an ASM.
For Venturers and Sea Scouts, the same rule applies at 21, too. They can remain YOUTH members of the Crew or Ship) for the rest of the charter year, even if they can't complete advancements or earn youth awards.
Paul S. Wolf, P.E.(Ret.) mailto:pwolf@usscouts.org
Secretary, US Scouting Service Project, Inc.