reset the alarm.  He stays out the night .

Deadbolts that you lock at night and windows that lock with a key so that he can not exit.  Yes, that is a fire hazard but you can have the keys in an easily accessible place that you can grab in the unlikely case of emergency.  If your SS objects that it is a threat to his safety, the question is why his safety is more important than yours?  The threat of being unable to leave in the unlikely event of fire versus a frequently OPEN door at night that any stranger can simply waltz through as you sleep seems pretty uneven.  He can cure the problem at any point by keeping track of his key and locking the house properly when he leaves. It is entirely up to him.  You should not have to sleep with one eye open every night to listen for him leaving to get up and reset the alarm.

The next time he sneaks out lock all of the doors and turn on the alarm.  When he returns he will be locked out and may even set off the alarm.  This will wake up DH and he can let SS in.  The next morning you say something like, "I got up to do ______.  Can you believe we left the door open and the alarm off?" 

DH sees himself in SS.  If you judge SS's aspirations DH will see it as you judging him.  DH has to deal with SS completely.  That means he chauffeurs SS around, opens the door for SS in the middle of the night when you are 'fast' asleep, and continually has keys made.  If DH suggests you do anything for SS you will always have an excuse for not doing it.  When DH speaks of SS, or SS starts his lying or storytelling, you will change the subject or smile and walk away.  Stop reminding DH of SS's birthdays, Christmas presents, etc.  All you have to do is disengage and let DH do it all.  If possible, separate all finances and only pay for 1/3 of the household expenses.

If ( and it’s a big if)  he is going to live with you, you need to set strict rules.  He breaks the rules and he is out.  Full stop.  It is not complicated.  This is total nonsense and you need to stop it now.

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Don't wait for DH to do it.  You do it now.

Why does he need to sneak out at night? If he's going to spend the night somewhere else then he should just do it! Make a rule about locking hte doors at 11 pm (or whatever time suits you) and anyone who is still outside at that time stays outside, anyone who is inside at that time stays inside. This is your home we're talking about, not a hotel.

SS18 no longer lives here as of yesterday and I'll be re-keying the house this week. My DH was a jerk to me and blames me for "forcing" SS to live with BM. I have little trust in SS, he would never hurt our animals but I don't trust who SS might bring over, and he's also left doors unlocked many times. He's sold items online for cash, and I have screenshots showing the photos SS took while in his room or our living room. Once I came home early and SS was leaning into the open door of a strange car in the driveway. I pulled up & SS kept talking to these total dirtbag-looking people in a junker-looking car for a moment but they took off. I asked who those people were and  why SS had them over when he wasn't allowed guests when me & DH aren't home. SS said they were just random strangers that pulled into our driveway to ask directions. Yeah, right. Total bullshit- it was a drug deal or SS selling stuff online for cash. 

It was a full out battle until I got my husband to understand that it felt unsafe to come home to a house that had been open all day. Stubborn OSS was locked out quite a few times before he got it and then he moved out. Win win.

One door can have a combo lock on it that can be reset if needed.  Combinations can be specific to the person so you know who came and went when.  We are looking into doing this w/DS21 due to similar issues. He doesn't sneak out but he is terrible w/keys.  

Web cams!  When  he sneaks out... get up, lock up and reset the alarm. He can sleep on the sidewalk.  Or better yet... he moves out now and this is no longer an issue.

The combo lock is a great idea as well.  Our new house has them and they make lost keys a thing of the past.  Best of all is that if he pisses you off... you can delete  his code without  having to call a locksmity to rekey the locks.

Making him gone is oh so simple.

Approach this with DH from what YOU need, and set YOUR boundaries. DH, when SS turns off the alarm and leaves doors not only unlocked but wide open, I feel unsafe. Anything can come through that open door -- people, raccoons, rodents. I'm not willing to live that way. If SS wants to go out late at night, he must set the alarm and lock the doors behind him. If he doesn't have a key to get back in, then he'll have to wait until morning.

And if DH won't tell SS what the expectations are, then you must. It's not optional, and it's not a request. You're telling him what WILL happen.

Would DH allow that type of behavior from a houseguest? From your family? No, of course not.