I've got mixed feelings on this one.
First and formost I believe we let way to many people out of jail way to early, but let's put that aside for a minute
I have a real problem with someone who is a violent felon out of jail being able to say to the police/probation officer/whoever "That's not my gun, that is my Mom's gun!" It is too convenient for them to hide their ownership by claiming it belongs to someone who won't tell the truth because of fear, love, or some misguided loyalty.
Also, while it may not be illegal for YOU, it may be illegal for him, or it may violate the terms of his parol.
Now, there is a big difference between a convicted felon entering a home for a visit, whatever, and there are guns there, and keeping guns at what is basically the felon's perminant address is entirely different
Further, we talk about personal responsibility, sheeple being ignorant to reality, etc etc. A fact of life is that you frequently must make tough choices, sometimes unfair choices. I think right now you are facing a tough choice, of being willing to give up a tool for self defence for a short period while the son finds a dwelling of his own, or tell son he cannot stay there.
Finally, while many criminals would have turned out the way they did with the absolute best parenting possible, many criminals actually had been parented (or lack their of) in a way that increase the likelyhood of them becoming criminal. Now, this in no way reduces the personal culpability of the criminal, but to be brutally honest, I don't know you from Adam, except that you raised a criminal. Now, it may not have a whit to do with you, but it may. Until I know better, I consider your instincts and ability to read people circumspect, this son may not be as you believe him to be.
Now, for my experience with felons and guns. I had an uncle, who in his youth was quite a partier. Other than being caught drunk, and spending the night in jail once, he had no runnins with the law, but did have a period of time he spent with a chaotic drunk unsavory woman, and they had a kid together. They seperate, mother raises kid...poorly. Something happens and mom is no longer able to raise kid, so she gets sent off to live with her dad. Now, he has almost no parenting experience, and gets a troubled teen thrown in his lap. Basically he had not the skills to redirect her in a more positive light. Yadda Yadda....age 17 she runs off, and has a long list of thug boyfriends. Gets pregnant by one, he commits some felony, of which I don;'t know what, ends up in jail. After about 3 years he gets out and reappears in her life.
Now, this uncle viewed my brother and I akin to the sons he never had. One activity we would do together every thanksgiving was to go out shooting. From these trips, I know he had a remington 12 guage semi-auto, 16 guage sidebyside, 20 guage over/under, .22 magnum revolver 4". He talked about bringing out a snub 38 revolver, but it was quite old, and only took 38 longs, not regular 38 specials, and he had no ammo for it.
So about 2 years ago uncle dies of heart attack. Daughter, grandson, and thug felon boyfriend move into home. Because of his felon past, they cannot keep the guns in the home. So the guns are given to my father to keep for the grandson (his grandnephew) BUT....the guns are only the 16 guage and the 20 guage, and the 22mag revolver. I suspect the other two were more desireable from the eyes of a criminal (who don't realize it is a 38 S&W not 38 special) and taht they were either sold to some unsavory element, or were kept illegally.
We have had to deal with this nightmare and it is my understanding they cannot even be in a home where there are firearms present. We entrusted the guns to a neighbor while he was staying with us. I don't have a gun safe. 
