It is very difficult to withdraw a plea, under any circumstances, in part because by attempting to withdraw a plea, you are telling the court that you lied when you pled guilty. It would be difficult to get a court to consider withdrawing a plea. Consult with a good defense attorney in your area if you wish to pursue this.
This response does not create an attorney client relationship and is offered for informational purposes only. Only a lawyer fully versed on the facts and circumstances of your case can properly advise you on the case. I am licensed to practice in Minnesota, not every state. You should always consult with an attorney licensed in your area on how best to proceed.
Your brother only has a year to appeal his conviction, pleading guilty and accepting sentence is the same as being convicted. You need to contact an attorney that handles criminal appeals, it sounds like ineffective assistance of counsel might be one avenue to go down. The appeals attorney will get the transcripts and look for other ways to appeal, it is a lengthy process so the sooner you get it going the better, Good luck to you.