How can my brother pass the B1/B2 visa interview if he is single and has been unemployed for years?

Please note that there is no official requirement to have "strong ties to home country" to qualify for an US visitor visa. The 214(b) requires applicants to overcome the presumption they intend to immigrate. Showing strong ties to a home country is one of ways to do so, but not the only one.

For example, someone who is rich and well-traveled will likely get an US visitor visa. This is even if all they do is travel the world, have not visited their home country for many years and have zero ties to it.

So to overcome this presumption he needs to think of what evidence he can bring which would prove that he doesn't want to immigrate to the USA, and would leave the country when his visa expires. Does he have any future plans and solid commitments anywhere else after the trip?

Also if he "has been unemployed for years" means he's doing so well financially that he doesn't need to work, this would also be an argument he is unlikely to immigrate. On the other hand, if someone else is supporting him, his visa might also be denied with 212(a)(4).