As I mentioned in my comments, I voted to close as too board because without more information, it is impossible for a reasonable person to answer this correctly because we'd only be guessing. We can't know why your dog is howling without knowing more information about your environment and without getting inside the head of your dog.
Now you did get an answer, and the answer sounds reasonable, but to quote part of that answer (emphasis mine):
So it can be response to some sound he hears. But we can only assume.
Ultimately that is the reason I voted to close. Any and all answers would only be guessing.
Now let's talk about improving, there is a slight situation here that can be discussed:
What could make my dog howl a lot in the night?
The question in the body of the post
What could make him keep howling during the night and what can I do to stop this?
You seemed to take offense to the fact that I told you to ask about making the stop when you mentioned it in your post. The problem is the title. Everything in the post is about why. Remove the why, focus on the stopping and I'll vote to reopen.