How long he is held, and how a probation violation is handled, depends upon the jurisdiction, and the resolution of the probation violation hearing. I've been called at 8:30 AM to be informed that a (former) client had been picked up over the weekend, and "can you be here in 45 minutes for a probation violation hearing" - if the court he is in works that fast, the matter could be resolved rather quickly. It is possible that your jurisdiction takes more time, in which case he may have the opportunity to bond out.

I can't advise you as to what to do on his behalf. For that type of advice, first thing tomorrow, consult a local criminal defense lawyer.