Your brother only needs to go to the solicitors' firm once to sign the necessary document(s).
In fact, if your brother is not able to come back to Hong Kong, the matter can be dealt with by 2 ways:
1. You can ask the solicitor to send the document to your brother and ask your brother to sign before a notary public in the foreign country and then the document has to be authenticated (your solicitor will tell you the procedure); or
2. You can ask your brother to execute a "Power of Attorney" in favour of a person he trusts. That person is the "attorney" of your brother and that person can sign any document for your brother. However, the Power of Attorney has to be signed before a notary public and has to be authenticated (just like no.1 above).