Making someone fall in love with you is no easy task and takes a lot of time. There's no guarantee it will work, of course, but there ARE ways to plant the idea of love in his subconscious.
Now, of course, manipulation is morally questionable at best. But I don't think there's any need to lecture you since so many people have done so already! :P
The most important thing if you want to go this route is to NOT be upfront about your romantic interest in him. That does not mean you need to detach yourself from him, but forget about being blunt, open, honest, etc. If he doesn't already love you, that will only drive him away. The key is to be independent, make him see that you're doing your own thing and don't depend upon him. Most men are naturally attracted to women who appear low-maintenance. Mixed signals are also a very handy tool; often times, confusion can be mistaken for feelings of love, and once that idea is there, it is a matter of wether he finds you genuinely appealing (which I figure he must since you're sleeping together). There are plenty of other subtle things you can do as well. A little bit of research mixed with your best judgement should help you come up with more ways to show him that you are deserving of his love.
The most important thing is to isolate him in these feelings; if he starts to show signs that he may be falling for you, it's important not to reciprocate them right away. Men love the thrill of the chase and the longer you deny him the comfort of knowing wether you love him back or not, the more he will want you. Again, this is something that takes time and is not true of everyone. Just relaying what psychology and experience have taught me!! :) Good luck!!!!