This is a serious situation and the consequences are going to be significant on multiple levels.
Work release is one of the most privileged statuses an inmate can have. It means the system trusted him enough to send him outside the walls with minimal supervision. Getting caught smuggling contraband back in is a direct betrayal of that trust, and the response from the facility and the courts will reflect that.
The immediate consequence is the SHU. Expect several months in segregation at minimum while the incident is investigated and disciplinary proceedings play out. Work release will be revoked on the spot.
From there, a transfer to a higher security facility is almost certain. Whatever low or minimum security placement allowed him to be on work release is gone. Smuggling contraband, particularly a weapon, is exactly the kind of conduct that results in a security level increase.
The two items carry very different weight. The marijuana is serious and will result in new charges, but it is the knife that creates the most exposure. Introducing a weapon into a correctional facility is a felony in virtually every jurisdiction. Prosecutors treat it aggressively because weapons inside prisons get people killed. The new charges from the knife alone could add years to whatever he was originally serving.
Whatever short sentence he had before this incident is likely a distant memory now. He needs an attorney immediately to assess the full scope of what he is facing before any hearings take place.