If your brother calls Delta, it's very likely they'll waive the basic economy restrictions and reschedule him after a quarantine period. It likely will not be even particularly controversial - I've had to make two adjustments due to COVID, and while both were on ordinary Main Cabin tickets, the changes were made without any add/collect of additional fare. In both cases, a supervisor was necessary, so be willing to be a bit pushy to ask for a new flight if the first agent you speak with isn't forthcoming with offering a change.

Delta is one of the most generous airlines in situations like this, and while they've actively advocated for a mass infection strategy in their public lobbying efforts, they're still pretty helpful and understanding with individual customers.

While public guidance has been loosened, the right thing to do is still unambiguously to self-isolate for at least five days (ideally until you pass a negative COVID self-test), and then mask with a high quality N95/FFP3 grade mask on the way home. If your brother is unwilling to do that, at least encourage them to mask - but note that they'll be knowingly choosing to risk infecting a whole airplane full of 200+ other people, which is something I certainly wouldn't want on my conscience.

As others have said, travel insurance can make this decision much easier. I've started purchasing an annual policy from Allianz - for about $500 for two people who travel frequently, it offers immense peace of mind, and they've actually been very generous in the claims process when we've needed to use the coverage.