The honest answer is that it depends entirely on the judge, and that judge is not going to be in a generous mood.
Here is the reality of the situation. When a judge grants probation, they are extending trust. They looked at the person in front of them and made a decision to give them an alternative to incarceration. A probation violation, even a first one, and even for something as relatively minor as a small amount of marijuana, tells that judge that the trust was not honored. The judge's response to that is personal and it varies, but it rarely starts from a place of sympathy.
The worst-case outcome is that the judge revokes probation entirely and orders him to serve whatever remained on the original suspended sentence. If there were two years of probation left to run, that could mean two years in custody. If there were six months left, six months. The length of the remaining original sentence is the ceiling for what the judge can impose.
The best case outcome for a first violation on a minor possession charge is reinstatement of probation with stricter conditions, possibly including mandatory drug testing, more frequent reporting, or a short jail stay as a sanction before being released back onto supervision. Many judges use a first violation as a warning shot rather than a full revocation, particularly when the violation did not involve a new crime beyond possession.
Where it lands between those two outcomes depends on the judge's history with similar cases, your boyfriend's overall conduct on probation up to this point, and what his attorney presents in court.
He needs legal representation before that hearing. And the marijuana on him when he got picked up is a conversation you two will eventually need to have about whether the risk was worth it.