You do know there are guidelines around these things. Can't just pop shots in quick succession like knocking back drinks. In Hong Kong, for Sinovac, minimum time between 2nd and 3rd doses is 90 days (unless immunocompromised, then 28 days). Barring some change to the rules, I'd imagine that your brother is SOL.
Here's the govt press release which documents the provisional vaccine pass arrangements.
The Provisional Vaccine Pass is valid within 180 days (i.e. six months) from the date of arrival. During the period, inbound travellers may carry with them or present the Provisional Vaccine Pass as a substitute for the vaccination record to enter specified premises under the Vaccine Pass. The Government has also updated the "LeaveHomeSafe" mobile application to version 3.2.3 for members of the public to save the Provisional Vaccine Pass to "LeaveHomeSafe" so as to facilitate scanning on entering Vaccine Pass premises subject to active checking.
Getting our fourth shot yesterday, there were two American girls directly ahead of us. Right after getting their shot, during the 15-minute waiting period, I guess they were reading the paperwork, and we're like damn, we didn't need to get another shot, we have a 180-day vaccine pass eligibility! They were berating each other for not having read up on it before. Had to chuckle. Anyway, it will do them good to have had presumably their third shot.