Ya know, I have a child with a developmental delay and found I had to let go of trying to figure out 'why'. It is what it is. Most physicians are careful about psychiatric patients. However, due to the nature of their disorder---- the risks outweigh the benefits of treating with medication during pregnancy. Seroquel is one that is used because its indications are for those that suffer disorders that are considered severe. Usually a lower dose is given. But doctors do a risk/benefit ratio and make the decisions after that.
I would say any number of reasons contribute to bad behavior in our kids. I would find out what the root cause is for your son (add/adhd, sensory, etc.) if anything and work on solutions for that.
Sandman has some good questions. good luck and peace
How old is your child and what is the child doing thats a behavior problem?