One thing is, be clear and specific. If you are looking for a place to do graduate study, be sure to ask for this. "I am looking for a place to do my PhD. Would it be possible for me to be a PhD student at your lab? Would you be available to be my supervisor to complete my PhD?" If the answer is hazy, or unclear, or non-committal, then consider that you will soon need to make clear commitments to apply to a university. You may need to pay out money to apply. You may have to forego other possibilities to apply, due to time and money.
His response might reasonably be to ask for your record so far, like transcripts or references etc. And if you have financial support of some kind. And all the mundane things.
If there is agreement to take you as a PhD candidate, but the specific research topic is not decided in advance, you need to decide if that is a deal-killer for you. Again, ask specific questions. "What possible research topics would you be willing to supervise me for?" Again, if the answers don't please you, or they are vague, you need to regroup and re-plan.
With luck your academic career isn't limited to one research question. So, if you don't do the first choice question now, you may be able to keep that "in your pocket" for future work. That's got some value. It is a rare PhD candidate who gets to do all the research stuff he wants without fitting in to some existing research program. And being able to "hit the ground running" with your own original ideas after you get the PhD is pretty good.
Any reasonable prof will understand that you need to make firm decisions so you can decide what school you are attending. And who is to be your supervisor. If he won't, or can't, be helpful on that, maybe he is not the right supervisor. Possibly it is not that he is a bad person. Maybe he is just busy and distracted. But "busy and distracted" now, while it is your only data, is still data.
