The short answer is they can hold him until the end of whatever sentence was originally imposed, and in some cases that is exactly what happens.
Probation violations go back to the judge who handed down the original sentence, and that is where things get complicated. That judge already showed leniency once by putting your son on probation instead of sending him to prison or jail. When a violation brings someone back into the courtroom, the judge's goodwill from the first time around is typically gone. They tend to view it as a clear signal that the earlier generosity was not taken seriously, and they sentence accordingly.
Since January is several months of sitting in the county already. That time may or may not be credited toward any new sentence, depending on the jurisdiction and the judge's order, which is something his attorney needs to be pushing on.
The violation hearing itself is what determines how this resolves. Unlike a criminal trial, the standard of proof at a probation violation hearing is lower. The prosecution does not need to prove the violation beyond a reasonable doubt. A preponderance of the evidence is enough in most jurisdictions, which makes these hearings harder to beat than people expect.
If he does not have an attorney working this, that needs to change immediately. The hearing outcome and whether that time served gets credited are both things a lawyer can meaningfully affect. Without representation he is walking into that courtroom at a significant disadvantage.