Can you put a hook for the key for the deadbolt as high as you can reach on the wall next to the door? And move any furniture (i.e. hall tables) that he might use to stand on? Maybe put one of those hooks that keep a door held back (usually on the bottom of a door) at the top as another precaution to keep it closed? Or if your door opens inwards, hammer a piece of wood at the top of the door frame that you can either turn horizontally so you can open the door, or turn it vertically so it extends over the door and DS can't (theoretically) open the door. These may not work, depending on the house, and furniture etc, but I figured anythign is worth a try.
I really feel for you hun - I can only imagine the sheer exhaustion you are feeling - especially now being pregnant again!
If it comes down to it, and it really is your last option, then maybe you need to go ahead with this - at least for a year or so until DS is old enough to understand. The only thing would be if he is that determined, he might find a way around the fence too!
What does your DH think about it? I imagine he's pretty busy running the property too - I really don't envy you at all!