Jorge's relationship advice is based on experience and observation. Let his trial and error be your success (hopefully).
How to Tell Him You Want a Relationship
Specifically, this article is about how to tell a guy that you want a relationship with him, and not just . . . er, relations. Of the carnal variety. Of the biblical sense.
It may be that you're dating the guy or that you've just started to get to know him, but, regardless, you've come to the point where you want some sort of advanced intimate relationship with him.
First, though, let's consider something: Do you want a relationship with him, or do you want to be able to say that you're "in a relationship" with him?
There's a subtle difference. Technically, you already do have a relationship with him, albeit one that is non-exclusive and "unofficial," so to speak. It may be that you already have an intimate emotional relationship with this person that has just gone undeclared, or it could be that the relationship is honestly merely physical.
In the former case, examine why you would want to make the relationship exclusive in the first place. Is it that you want to "capture" them and make them yours finally so no one else can steal them away? Is it that you just believe this is the logical progression that all people must follow—that if they get on well and have an emotional connection, the ideal end to this is some sort of official relationship?
In the latter case, where the relationship is purely physical, do you really think that being "in a relationship" with him is going to suddenly make it emotionally deep and not completely physical, merely by virtue of the fact that it's exclusive and no one else would be in either of your pants?
This isn't an attempt to convince you out of it or anything; it's just worth it to examine one's motivations in any kind of circumstance. Anyway, so if after considering these things, you want to proceed, these are the things you may want to keep in mind:
Decide What Kind of Relationship You Want
Know what kind of relationship you want, and let him know. A completely exclusive one, an open one, a polyamorous one, a long-term one, a short-term one, D/s one, etc.
If you have any expectations and insist on having them, communicate that to him. If you have no expectations and are willing to just allow the relationship to be what it needs to be, then communicate that to him. Either way, it's better for the two of you to be on the same page ahead of time than to discover unpleasantly that you had two totally conflicting ideas on how this was going to be.
It's probably also not a good idea to say you want one kind of arrangement in order to get into the relationship when really that kind of relationship wouldn't make you happy, and you were just hoping that he would change his mind.
Ex: He wants an open relationship, and you can't stand to not be the only one, but you hope that he'll come to love you enough to change his mind; he wants someone he can tie up and whip, but you're not into that and hope he'll spontaneously change his mind once you get together; etc. If you want to take that chance, go ahead, but part of wanting to be with someone is accepting that they want what they want.
So basically, above all, be as honest and up-front as possible. Don't go into the relationship with the notion that you want to change him. Yes, this is basically a cliché, but don't. People don't change for other people most of the time; they'll only change for themselves.
How to Ask for a Relationship
Now that that's out of the way, how to actually ask him for a relationship:
Just be straightforward. There's nothing better than that. Tell him that you like the connection that the two of you share and that you feel it's special. Tell him that you'd like to deepen the connection even more and that you hope he feels the same.
If he doesn't respond to this kind of sincerity, then he either doesn't want to be in a relationship (and maybe you should seek one elsewhere) or he's kind of an asshole.
Anyway, I wish you luck!