Unless the judge waives the fees while your son is in jail & rehab, then they will continue to accumulate. However, most judges, in my experience, waive the fees. In fact, some judges allow rehab time to count as community service time. It is different in every court across the state of Texas.
Your best bet, since your son is locked up, you should call his probation officer. Even if the officer cannot give you direct information about your son's case, you could ask a hypothetical question about people in general in your son's position. (There also ought to be amended conditions of probation which your son signed & a copy of which should be in the clerk's file which should address the fees, etc. It will state what his current conditions are in full.)
If your son has already been assigned to rehab, then the appointed lawyer has finished his/her job and no longer represents your son. Moreover, at this point, without a motion to revoke, motion to adjudicate, or new case pending, your son is not entitled to have another lawyer appointed.
If the court appointed attorney has been removed have your son ask for another court appointed lawyer. Your son needs a lawyer to speak with the probation office and prosecutor.
There is a defense for failure to pay probation fees because of an inability to pay. However, the defendant has to put on evidence to support an inability defense.