Am I being reasonable?
It's very reasonable to want to gather all your offers before making a decision. Sometimes that approach is viable, sometimes not.
What's a good way to express this to my boss to wait as long as
possible for me?
Hopefully, you already have a good relationship with your boss.
Go to her/him and say something like "Hey, boss. This is a really difficult decision for me, and I want to make sure I get this right, career-wise. I've really enjoyed working with you, and I really appreciate the offer. I think you can understand that I'd like to wait a bit before giving you my answer. Can I get back to you in X weeks?"
Often, that will work. Employers of interns know how this end game plays out. They clearly like you, and would like your commitment to join them.
Sometimes your current employer will just be trying to add you into the mix after a successful internship. They will expand their headcount to include you, should you accept.
But other times, they have you earmarked to fill a position that must be filled at a particular point in time (in order to complete a project, or start a new one, etc). If that is the case, they may need an answer sooner than later, so that they can pursue other candidates in the event that you decline their offer. You might have to make a difficult decision with incomplete information - so it goes in the working world.
With interns I've hired, I have encountered both situations. Some interns have accepted my offers, others have declined. If they were good workers, they always got great recommendations anyway.
A quick discussion with your boss should get things moving in the right direction one way or another, and will likely get you your desired positive recommendation in either case.