It’s very simple if he has not submitted all the required documents to them, he can resign and forget about the company.
2.If he has submitted all the required document he can still resign , as per Qatar Labour law no (14) of the year 2014; Article 39; either of the parties i.e the employer and the employee can terminate the contract during the probation period with just 3 notice or so.
In summary: there is nothing to worry about he can resign, because your brother is still under probation period and he has the right to leave; just tell him to give them writing notice.