Translation:The boy behaves badly because he has not had breakfast.
I put - The boy behaves himself badly because he has had no breakfast. It is also considered wrong. As a life long Brit I consider this and your reply to be correct. In British English there is more than one way to skin a cat. Which makes the language a lot more flexible and probably contributes to its popularity.
German would require inversion in this situation, but Dutch does not have a strict rule one way or the other. As said earlier, both are correct!
Yes it is a verb by itself, you may check the "reflexives" section. When the verb already has a prefix, you don't need to do anything, you just keep the prefix. See rule two in the following page: http://www.dutchgrammar.com/en/?n=Verbs.Re12
I am not sure what the end of that verb would be in present perfect. But I guess it would be "gedraagd".
Edit: I was wrong. Its past version is "gedragen". See the comment of xMerrie below.
It's unnatural in English to say "a breakfast" in this context. The only time I can think of where we would use an indefinite article before a meal would be something like, "They are serving a breakfast consisting of ...," or "The group is hosting a breakfast prior to the conference." But if we're talking about someone's eating (or not eating) the meal, it's just "to have breakfast."
The English sentence sounds wrong here, the use of behaves and because just doesn't gel. The boy behaved / is behaving badly because he has not had breakfast flows better as you are explaining why something happened or is happening. To me using behaves implies it is conditional i.e. the boy behaves badly when/if he has not had his breakfast.