I'm Lucy, and I'd be happy to answer your questions today. I'm sorry to hear that this happened.
It is extremely difficult to withdraw a guilty plea. The standard is that a person will only be able to withdraw a plea if they can establish that their will was overcome, and that they did not voluntarily plead guilty - meaning that someone forced him to do it, and he wasn't able to think about himself. It's a high standard, and requests to withdraw pleas are rarely granted, but he does have a right to put in the request.
It is even more difficult to withdraw a plea after sentencing than before. He has a better chance if he files as soon as possible, before he is sentenced. The standard is the same, but suddenly the judge is also wondering if he only filed the Motion to Withdraw because he didn't like the sentence. He can ask the original lawyer to file the plea, or he can hire someone else. It might be worth calling the public defender's office in your area to see if they are able to assist with this type of thing.
If a judge grants the plea withdrawal, he will get a jury trial. The district attorney will probably stop all plea negotiations, and if he ultimately is found guilty, the sentence will be more harsh than whatever he agreed to. It's important that he understands that, because the issue at trial isn't whether he actually did it. It's whether the district attorney has enough evidence to convince a jury beyond a reasonable doubt that he did it. His first lawyer thought they did. It may be worth sitting down with a second lawyer who can review the evidence before he decides if he wants to take that risk.