I'm going to do the best I can with this, but realize that I do not have all the details of this case. I don't know your son's age (if he's under 17 in Texas he's a juvenile, and if he's 17 he's an adult for the purposes of criminal law), and I don't know his background as far as the law is concerned. I also don't know any criminal history your son might have that would change my opinion. I also don't know if your son actually did what he's accused of doing.
That said, you can probably still go back to the attorney and ask if the court has offered your son deferred adjudication. Essentially, if he's never been in trouble with the law before, he's probably eligible for deferred adjudication, which is like probation. Your son pleads no contest and doesn't re-offend, and possibly pays a fine, and after a given amount of time the charge will be taken off his record. This would especially be a good option if your son is a juvenile.
You can fire this attorney, and if you're poor it's possible that you can ask the court to appoint a lawyer for your son. (If your son is an adult, he'll have to ask the court himself.) Or you can ask the attorney to explore options other than going to trial, and to find out what deal the court is willing to offer and what consequences it would have for your son's record. But seriously, you don't "have" to go to trial. And you certainly don't have to pay a lot of money to an attorney you're already unhappy with, although it sounds to me like you all have had a misunderstanding here as much as anything.
Call the attorney and see if you can change things. Find out what your options are. And stay calm. It probably isn't too late to get through this with a minimum amount of both money and consequences, depending on the surrounding circumstances.
Please keep in mind that they lawyer's responsibility is to do what your son wants since he is the client. I realize that this may seem awkward since you may be the one paying for the attorney.
If your son is innocent of these charges and his attorney can not convince the prosecutor to dismiss the charges his only only option is going to trial. A responsible lawyer would never have a client plea guilty to something that they did not do. Your son may have told his lawyer that he is not guilty of the charges leaving him no other choice, but to set the case for trial. Most importantly, a theft conviction is considered a crime of moral torpitude that would be with him for the rest of his life.
The issue about the fees is another story... I would hope that the attorney entered into a contract with your son for his services before he was hired. Typically, a criminal contract has one fee for the case if it does not go to trial and then another additional fee if the case needs to go to trial.
On the other hand, if you son is guilty he may request a deferred adjudication. Luckily, a class "c" deferred is eligible to be expunged upon succesful completion. I have explained how this works at http://www.houstoncriminallawjournal.com/2008/11/articles/houston-criminal-attorney/expungement-versus-motion-for-non-disclosure/.
Good luck to you.