Your new best friend may be using the term "partner" because he already has one, so I recommend slowing it down just a bit and finding out more about him before sharing your feelings of sexual attraction. Why not ask if he wants to hang out on a weekend or one evening? That'll give you a more relaxed environment to find out more about him, including his love life -- who he's dating (if anyone), how serious the relationship is, how long he's been out, etc.
Naturally, you, in turn, can share similar information -- your dating history, what type of guy you find attractive, how long you've been out, etc. Because of his frequent reference to the word "partner" I would make it more of a bonding experience in which you compare experiences and get to know one another. Even if he's madly in love with this other guy, maybe it's not all bad. With you being new to the school, perhaps they can introduce you to other gay young men who are potential love interests.