L., there are a couple of things you can do.
First, you can buy a lever lock that will lock those lever type doorknobs. You have to install it, but it's really easy. I bought mine at Walmart or Target.
Second, if you don't like the idea of locking the door, you could also temporarily replace the lever doorknob with a round doorknob (I bought one at Walmart for about $8) and get one of those things that just makes the knob spin without catching, so the baby can't open the door. Of course, one of my sons learned to open that around 23 months, so I can't guarantee it. But it makes the door fine to open from the outside, just not by a small child from the inside.
Also, you need to make sure that your outside doors are locked securely so your son couldn't make an outside trip during the night. We had one night wanderer when we moved into our house, so we put a hotel-style lock at the top of our front door in order to prevent him from opening the door.
Hope this helps! I know how exhausting and frustrating it is to have a night wanderer. Good luck!