How do you manage the front door at your LO’s home?

Leaving it ajar as in not fully closed? Or do you mean closed but unlocked? Can you please clarify?

I'm interpreting your question as he is leaving the door somewhat open. If you can show him that there's ways to keep the door shut and locked until someone comes (like suggested in the other responses) and they can let themselves in and out.

But if he doesn't remember this solution, then maybe install a home security alarm system that alerts YOU that the door is not closed and calling him to shut it? Or is there a neighbor in proximity who you can enlist to go over to shut it for him?

We got mom a keypad lock. We were able to give the combination to helpers and to 911 when that was necessary.

We placed a lock box on my parents front door to safely allow caregivers, therapists, etc. to come and go.
It has been wonderful and is exactly like the ones real estate people use. You can find at Lowe's, Home Depot, etc.
You can change the code occasionally if needed.
Very easy to manage. Can also be used for food deliveries.