Q1 how is anyone sure of anything subjective and personal? They can't be. You can't prove or disprove feelings and opinions. The answer is just...good enough for you. Remember, identity is totally self-defined. It's not diagnosable. No one can or gets to tell you you're wrong. If you say you're aro, then you're aro. Identity isn't about an objective standard of correct. It's just about you.
Q2 that's okay. Coming out is difficult and scary! Do it at your own pace, whatever that means.
Q3 I would just be straightforward and lay out the facts. I would also have the coming out discussion separately than asking if he wants to have casual sex together/explore a qpr/whatever it is you're thinking, though. Those are two different convos and I know it's nice to just get all the scary stuff done at once, but I would give you both time to process each conversation at separate times. You could also ask him to be friends with benefits without coming out. You're not obligated to come out first or anything. :)
Good luck! Feel free to hang out here and ask as many questions as you want.