I am a Probation Officer. My best peice of information I can lend, as others have already, is to contact your son's PO and ask him or her directly. Other information I can lend on the subject should not be taken as legal advice, but simply as my personal and professional experience.
I generally encourage the family or cohabitants of offenders to store their weapons at a different location. Especially if the offender has firearms/dangerous or deadly weapon conditions on their supervision order.
As Seaweed02 has said, parole/probation checks are usually confined areas of the home the offender has access to, however it should also be noted that some departments allow a plain-view search of the other rooms as a matter of officer safety. This is not to say that items found in these areas can or will cause a violation in the case, however it is information worth having.
Keeping them in a locked safe away from his room or common use areas of the home with a combination he does not know can work. However this is sometimes problematic because proving that an individual does or does not know a combination in next to impossible.