I know many people have started out “fostering” for a shelter and the foster soon became one of their brood. It’s not easy to find GOOD homes for rabbits, if it were, there wouldn’t be so many shelters/rescues. If you were to get this bun, I would go into it expecting it to be permanent. You would have to neuter him yourself, and then begin looking for a suitable home. This could take months. It’s different then if you were actually working in conjunction with a rescue where they would be providing vet care. Do you have one nearby that you can contact? I know you had to travel a bit to get Gracie and Archie… It’s really up to you, but it’s something you’d have to seriously think about. The bun would also possibly need to be housed somewhere away from your pair as a new, unaltered bun can disrupt the bond. You still have Archie’s old cage, right? So you could house him in there, at least temporarily.
(I’m assuming the bun is unneutered if he is free, 6 months old, and they *suddenly* need to rehome him. It sounds like he might be in full-blown puberty and they want to get rid of their “problem” instead of fixing it.)