Why would you need to prove that you "legally" had the money available to lend him?
The federal reserve lends people money all of the time without having it. And they charge interest, too.
I would look deeper into ANY company, before I divulge any financial info to them.
Perhaps they are concerned that (since you are brothers) he didn't "help himself" to the check and signed your name to it.
I would offer a notarized letter stating that you, did indeed, lend him the money, but not any account info.