They are not exactly the same, although they are both antipsychotics. You can always ask your pharmacist to explain how they are different.
or your doctor which should be the person you pose these questions to - sorry but drug interactions and reasons for prescriptions are questions which belong with a medical professional. Both of these drugs have different effects and side effects and act differently in different doses - for example haloperidol and mitrazapine are both sedating and from what I know the higher the dosage of haloperidal the more severe the sedation becomes which may be why he is using the combination but thats the pdoc's job to work out - drugs are a balance and getting the right cocktail takes experience and some measure of experimentation, sadly its not something you can research on the web although the web is a good place to find information about your drugs so you can be educated and ask the right questions.
In this case a google search for haloperidol and zyprexa can give you information on both drugs and how they work and their side effects so when your brother sees his doctor he can ask the right questions.
Not to put you off in any way, but Zyprexa was the worst drug I ever took.
It caused ME, (you sound different) to gain severe weight gain, have every side
effect possible. Hope my views don't change your mind, after all they are