The question "Why are you here?" is much too ambiguous.
If your intent is to ask "Why (or how) were you created?" please do not ask that, at least not until your guy has been in therapy with a good therapist for a few years and is willing and able to talk freely about his childhood trauma. Many alters don't actually know or are not able to speak the answer to this and those who do know are likely to be all too aware of all too much raw, unprocessed trauma. Some alters are created in infancy or early childhood and unless they have been awake and aware a lot over the years since, they have no vocabulary for what happened to them. They literally do not know the words. By asking them this question all you would be doing is taking the risk of triggering a flashback. Such a flashback is likely to be further traumatizing.
If your intent is to ask "Why are you here with me now?" consider how it feels to be asked that question by someone, especially someone you don't know all that well. Kind of off-putting, isn't it, like you're not welcome. And again, some alters won't have any idea why they are in executive control just then; they didn't choose to come out, some other alter(s) pushed them out front.