In the eye's of the law, he should be found guilty of improperly storing his firearm and the ammunitiion. That's it.
In his heart, he knows he's guilty of being a horrible parent and of contributing to his own son's death, and ruining his other son's life. That will haunt and torture him more than any other sentence he could receive. Leave the family alone, let them grieve. They've learned their lesson.
I guess if I'm the judge, I'd have a heart. I'd find him guilty of improper storage, call it "suspended sentence with time served" and threaten him that any further gun infraction besides putting one up to his own head and pulling the trigger (which he'll probably do out of shame for ruining his family) will land him in the pen.