As I best understand it, those under felony indictment cannot possess firearms. I may not agree with this law in every case as it seems like deprivation of rights without a conviction. But it is the law. And this is one of the rare cases where I'd impose some legal liability on the folks if their son gets ahold of one of their guns, absent reasonable efforts to secure those guns.
I don't know that guns should have to be removed from the home. A safe to which the son doesn't have access ought to be sufficient.
On the other hand, one person dead, three wounded, and a first degree murder indictment is no small matter. One hopes for the truth to come out at trial and some sense of justice to be achieved. But in the meantime, some kind of restrictions on the person indicted don't seem unreasonable.
If his folks still own a gun store, they should have a fine location away from their home where they can store their guns. Even if they've sold the gun store, they probably have plenty of good contacts who will store their firearms for them. Or, their son can stay somewhere else, probably in custody at this point. Kind of sucks, especially if it turns out to be legit self-defense. But not unreasonable either.