He can be accused of violating the terms of parole, and have a hearing on that revocation. That process is somewhat like a criminal trial, except that it is before a Parole Board officer instead of a judge, the defendant has fewer rights, and it is easier for the State to revoke parole than it is to convict him in a true criminal trial.
The new charge can also be prosecuted as a separate offense, with a new sentence.
An experienced criminal defense attorney will familiar with both parole revocation and the new charge. He has the right to have an attorney, at both proceedings.
Some basic information on the Texas parole revocation process is available here: http://www.tdcj.state.tx.us/bpp/revocation/revocation.html
Generally speaking when a person is on parole and gets another felony charge things go along these lines.
A new felony charge typically results in the parole board issuing what is called a "blue warrant"...a warrant seeking to hold the parolee in jail until a decision is made about whether to revoke the parole, impose some sanction short of revocation of parole, or continue the parole.
The decision whether to revoke is typically not made until the new felony charge is disposed of. This generally means that there is some chance...not a certainty but a chance...that if the new felony ends up being dismissed or reduced to a misdemeanor, the parole may just not get revoked.
What this means is that the parolee with the new felony charge needs a capable criminal defense lawyer who will examine the new felony for any weaknesses that may be used to push for dismissal or reduction to a misdemeanor.
Another issue that comes up is the interplay between the remaining time the parolee will owe on his or her original felony (the one that got paroled) if that parole is revoked AS COMPARED TO the sentence that the defendant gets on the new felony. Sometimes, depending on the strength or weakness of the new charge, even if conviction of the new charge is unavoidable, a low ball plea deal can minimize the damage and the time that the parolee actually ends up spending back in prision.
Every case is different, so I cannot advise you specifically about your son's case without doing a complete evaluation of that case.
Answers on Avvo are for general information purposes only and should not be relied upon as legal advice. No attorney / client relationship is created by providing this answer. For specific advice about your situation, you should consult a competent attorney of your choosing.