Probation searches are not limited to drug offenders.
The court can impose reasonable conditions on someone who is on probation. Your boyfriend's probation terms apparently include some sort of condition requiring him to submit to law enforcement searches of his car, home, person and other property.
The search can be for stolen property, burglary tools or drugs, depending on the offense.
Because you live with him, they can search any part of your house that he can access.