The fact that he said "girlfriend" to you suggests to me that though he may be interested in you he is still attempting to let you know his 'situation' and I bet that the reason why he nearly kissed you instead of kissing you is because he is struggling with his possible desire to get involved with you and dealing with the fact that he is already involved. If this is really what is happening, there are other possibilities which I will explore below.
Now I know to you that 22 is so old and mature that he is an adult. To me (nearing 40) I see him as a young "boy" who is neither mature nor should he be expected to be fully mature especially when it comes to matters of the heart. I still do stupid immature things and I have more than a decade on him.
Humans are complex beings in that we can entertain the possibilities of dating another when in a relationship with another. We can also flirt and be nice with the unspoken suggestions of further happenings while not intending to put across that message. That is where the other party must take care that the glint in the other's eye is not a reflection of the beam in their own. This is to say that we are not reading into something more than what is really there.
You said "
...he has almost kissed me once..." I get this image of an attempt but somehow his lips missed and hit the wall behind you or something along those lines.

Not being there to witness this event I have to wonder if this was more of a pregnant pause which allowed you to read more into what is going on that what actually took place - a possibility and if I am wrong so be it.
What it comes down to is this:
1. Did you ever ask him if he had a girl-friend and he said "no"?
2. Has he hid the fact from you by circumnavigating tricky questions?
Sounds to me when put to the question he came right out and said "My girlfriend" you do not report a hesitation or him saying something like "Just my room-mate" to hide his involvement with this person.
As you know I am gay - That fact has not stopped plenty of women from thinking that my most innocent hugs and smiles were saying more than "lets be friends" Even observers have mis-thought that my gestures and what I said carried with it more meaning than I meant. This has lead to some embarrassing moments for all parties concerned.
I have also made the mistake (on more than one occasion) of thinking another person was putting out a message that was not so - part of it due to my infatuation with them, partly due to my inability at times to understand the subtle differences between friendly flirting and pick-up tricks. The down side to the later is I also missed out on possible relationships because I assumed that somebody was just being flirty not understanding that they were sending out the "real signals" that they were really interested in me.
All in all what I am trying to convey here is that there are two sides to every story. It is highly possible that he is interested in being your friend but is still committed to his girlfriend. It is possible that both parties are putting out the wrong signals that are not their intentions.
It is also possible that he is interested in you and is struggling with the situation by doing his hardest to be faithful to his girlfriend but is wrestling with his desire to get to know you better. If his relationship is on rocky ground it could be causing him even greater conflicts especially if he is weighing the relationship he is in already and attempting to decide if its time to cut bait or if he will stick around a little longer.
Since you may not have had the chance to be in his shoes I am offering up these possibilities so you can have a better understanding of what could possibly be happening so you can decide exactly how to proceed with your next step which logically would be confronting this issue and asking point blank if there is something going on.
However, it is still possible that you and your lady friends are reading more into what is really happening. If you feel that this is the case then you may not want to enter that conversation because you may be embarrassed – which is a valid feeling and is a natural thing. In fact being in error in matters of love is part of the human experience.