Honesty is best, even at his young age. Dont give him details, explain it to him on his level. Say something like "You know when you do something bad and I have to ground you for a week and you can't play outside? Well daddy did something really bad and he's going to be grounded, in jail, for a long time." It's best to hear it from his mom, someone he trusts rather then finding it on the internet later. And I whole heartedly agree with Cakegirl, your son is 2nd grade? Let the counselors and teacher know, they are your first step to telling you if there's a change in his behavior/grades.
Good luck to you and I'm sorry for your troubles
Good luck!