My 8 months old westie only poo and pee at the right place when we are around the apartment.

Once we go into our bedroom to sleep in the night, we close the door, or if we go out of the house, he will poo/pee at all the wrong areas.

He usually poo/pee at the same place, the wrong one, when we are not in view of him.

What is wrong with him?

Nothing is wrong with him. You are expecting way too much from an 8 month old puppy.

Purchase a crate and put him in it when you go to bed. That will keep him from wandering around your home and pottying wherever he feels like it.

Don't expect your dog to be house trained 100% until at least 2-3 years old... especially since he is a smaller breed. Small breeds have a tendency to have a hard time learning the ropes of house training...

If you cannot watch him, put him in his crate. Crates keep good dogs good. They keep messes from happening and the keep things from getting destroyed.

Now is also the best time to start obedience classes. Trainers will work with you and guide you through tough problems.