Maybe it won't. Wait and see as these tests are not foolproof. If it's positive I would throw myself at the feet of the probation officer first. Maybe he'll just be reprimanded. Worst case is his probation be revoked and he would have to serve whatever the original sentence was.
If your brother's test turns out positive, then the probation office may talk to the judge about allowing him to take an alcohol awareness class. That said, the judge could also agree to a request to file a motion to revoke the probation. If the test comes back negative, your brother needs to be aware that not only can he be revoked, but his probation can also be extended and he could be given a number of additional conditions including in-patient treatment, community service, jail therapy, etc. Being this close to the end, he needs to tow the line and do what he is supposed to do.
As for immigration, I do not handle immigration cases I am not well read in the area. However, I have been told that the fact that as long as the underlying case does not carry a potential jail sentence of 1 year or more, then your brother is fairly safe. (The rules can change at any time.) If your brother is on DWI first probation, then he is safe. If he is on an enhanced probation which could subject him to a year in jail, then regardless of whether he gets his probation revoked, he is potentially subject to deportation.
Again, this information is for educational purposes and I am not positive particularly on the immigration issues. The information I shared I have gotten from other lawyers who may or ay not be correct.