Sounds like your attorney may be in a little bit of a bind.
No, generally you can not bail someone out when there is an INS hold (will technically you can post the bail but the person just won't get out).
If your brother didn't sign a contract and the attorney took 3500 without a contract (assuming you didn't sign a contract either), then the attorney has a challenge in keeping the money if you decide to fight his fee.
Generally an attorney MUST have a written contract if the fee is $1000 or more.
Good luck with the situation.
Well, maybe Mr. Williamson is more of a gentleman than I am. What you are describing is very unethical and should be reported to the California State Bar. You could also demand a refund of part or all of the money paid.