Your in an emotional tornado right now. So many different emotions running quickly it's hard to process any single one.
Safety for you and your son is priority. Keep in mind that he's going to be nice and sweet after he's had a fix and nasty and angry when he's not. Desperate addicts are dangerous and great manipulators, don't forget that.
He's already stolen from you, baggered you into signing some dumb piece of paper not to press charges against him, which by the way wouldn't hold up in court anyway so you need to really but up your guard, keep your son safe.
Focus on your sons well being, how's this affecting him? His emotional state? What he went through is traumatic and scary, embarrassing to be at a police station so make him your top priority especially in your thoughts.
Addicts are very resilient, jail would be the safest place for him and counseling for you snd your son the safest place for the two of you.