Motion to revoke

If your son is sitting in jail while he is awaiting a hearing on his motion to revoke probation, then any of the time that he spends in jail can count towards his eventual sentence.  This assumes that the judge revokes his probation and sentences him to jail/prison time. For example, if your son is on probation for a state jail felony offense and the judge revokes his probation with a sentence of one year in state jail, then he can get credit for any time in jail.  If he was in jail for six months, then he would only have to serve another six months in jail.  (The same concept would apply even it was a misdemeanor.)
If the judge does not revoke your son's probation, but instead, sentences him to jail time as a condition of his probation, then the judge is not required to apply his time in jail towards the time being served as a condition of probation.  "Jail time as a condition of probation" is basically where the judge has a defendant sit in jail and think about their choices-- also referred to as "jail therapy."  Some will give them credit to make them exhaust their credit.  A defendant cannot use the same credit for time towards probation jail time and then use the same credit for an eventual sentence.  Thus, if it gets exhausted on "jail time a condition of probation", it cannot be used later to reduce prison time should he ever be revoked.