Your facts are conflicting. You said he got 7 days (jail?) for the violation but then say he never got served with a VOP warrant. If the judge sentenced him to 7 days on the VOP, then there will not be a warrant for the VOP because he would have already been sentenced and thus no need for a warrant. On the other hand, if you misunderstood the seven days and he has not been sentenced on the VOP, then there will be a VOP warrant coming very soon. If he was sentenced, you should bond him out so that he can fight the charges while he is out as opposed to sitting in jail.
If you are found guilty of VOP you can receive all the way up to the maximum sentence for the original case for which you were placed on probation. You can receive the same amount of incarceration time as probation time for each level of offense. For example, 3rd degree felonies are punishable up to 5 years in prison or 5 years probation and a $5,000 fine. If your son can bond out he should, because he may be held on a "no bond" status once the VOP is filed.
You should bond him out and hire an attorney as he could be facing jail time for his VOP.
THESE COMMENTS ARE NOT LEGAL ADVICE. They are provided for informational purposes only. Actual legal advice can only be provided after consultation by an attorney licensed in your jurisdiction. Answering this question does not create an attorney-client relationship or otherwise require further consultation.