Building on what the other commenters said, I think you and your husband have a decision to make.

If your brother came and said to you that it turns out he can't pay you back, possibly ever, would you break off the relationship? If yes, by all means, have that "firm conversation" with him. That would be the likely outcome.

If your answer is instead something along the lines of that you'd always want to continue a loving positive relationship with your brother, I'd advise a compassionate conversation rather than a firm one.

It could be something like, "We realize things aren't going as well as you expected when you promised to start paying back within six weeks. That's ok. In fact, we'd like to modify our agreement.

"From our side, if you never pay back a cent, we'll still love you. From your side, pay back whatever works, whenever works, even if that's nothing ever. Plus, don't let this debt get in the way of our relationship."

Of course, next time he asks for a loan, if he does, I'd advise saying no to the loan, but gifting him whatever makes sense to you.