First, the jail doesn't decide whether he stays in jail longer as that is a decision reserved for the judge.
If his restitution is in collections, it is unlikely that he will have to go to jail for that since it appears to be at a phase where the court has already entered judgment against him. Yet, if there is an active warrant for failure to comply with the court's order to pay restitution, then he will likely be writted (taken) to the county in which he owes the restitution. After going to that county, most judges just reduce it to a civil judgment against the defendant and let them go. He will still have to pay any restitution. In fact, a lien can be taken out, his wages could possibly be garnished, etc.
If the court orders him to pay and he does not, then he could be held in contempt and technically could be put back in jail. From what I've seen as a former prosecutor, 9 times out of 10 the judge will just reduce it to a civil judgment against him. Now, to clarify, if indeed he gets out before this matter is handled in court, he may be incarcerated until his court date if he has a warrant out.
Bottom line: the judge can do whatever he/she wants. If the judge wants to make him do more jail, he will have to BUT any jail time is likely to be concurrent with the jail sentence he already has. this is not child support, it's just restitution. However, depending on the amount owed and for how long, the judge may decide to give jail. however, that's really, really unlikely if it's been sent to collections- it sounds like it's already been reduced to a civil judgment against him.
Hope this helps.
Linda Lee, Esq.