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I am a solicitor.
Your brother would appear to qualify for ILR under the new concession but he should apply before he turns 25 years, see here: https://www.connaughtlaw.com/early-ilr-concession-policy-for-children-and-young-adults/
May I help further?
You need to apply ASAP before his 25th birthday under the concession, do not worry about the 28 days rule as the concession overrides that.
All the best
He should have spent at least half his life continuously in the UK in addition to the 5 years limited leave to remain; https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1028126/Concession_on_longer_periods_of_leave_and_ILR.pdf
There has to be a request for ILR, so I would say apply for ILR.