Translation:The boy behaves badly because he has not had breakfast.
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."