Your brother can appeal his probation revocation. I advise that your brother (or you on his behalf) hire a criminal defense attorney (or possibly apply for a public defender), and provide all documentation surrounding the probation revocation.
A licensed attorney can appeal your brother's revocation (you cannot practice law unless you are one). However, the appeal will do absolutely no good unless there was an error committed by the trial judge. If your brother was on first offender probation, the judge was authorized to revoke his first offender status and resentence him up to the maximum sentence provided by law for the felony for which he was serving his probation sentence. As long as 6 years was within the maximum sentence for the crime, there was no error for the sentence. On the other hand, if your brother was not on first offender probation, the most he could have been revoked for would be 2 years. In that case, revoking 6 years would be an error.
If you think that an error occurred, you need to retain an attorney ASAP. Your brother has a very limited time in which to file in order to preserve his appeal rights.
Short answer to your short question: yes, normally probation revocations can be appealed if they are not agreed upon, and this does not sound like that situation.
You definitely need to provide more information, not only about your brother "catching" a misdemeanor, but also about the sentence that led to the probation violation. Important details will also come from the procedure by which your brother's probation was revoked. It is important to remember that he has thirty days from the date of the decision to revoke him to file a notice of appeal or motion for new trial with the court. That is what starts the appellate process in motion and if one of those is not filed in that time frame, he will probably not have the opportunity to appeal the decision.
That's just scratching the surface, really, and I agree with my colleague above that your best next move is to consult and attorney who has experience in criminal appeals and post-conviction work to figure out how best to help your brother. As I mention, the clock is ticking, so don't delay.