There could be a physical reason you don't feel heat for your boyfriend. I am very into Science and sociology so here's an answer form that perspective: Humans are attracted to features and qualities in others that tell about their genetic code, the way 2 people's genes intermingle decide what body shape the child will be prone to, shape of face, possibility of disease, some personality characteristics, as well as other things. It is possible he's not a great genetic match for you and your body being aware of this resists giving you the positive stimuli/encouragement to mate with him.
A woman's body has a surprising amount of control over who she chooses for the father of her children. Did you know that a woman's body can hold sperm inside for up to 72 hrs slowly releasing it toward the egg for greater possibility of impregnation? Alternately, she can reject the semen not allowing it to pass the cervix or impregnate the egg, if her body determines that he would not be a good choice. Also, in a study of the body's ability for selection: they took two different men's sperm, mixed them in a lab and injected them into an ovulating women. They discovered that her body rejected only one of the men's sperm not allowing any of it to cross the cervix, and allowed the other man's to cross and reach the egg. WOW, the power a woman!
Now, let's say your body doesn't think you too are a good match, well so what. Some men that I have been very physically attracted too were not great matches in a relationship - but the sex was great :) And visa versa. I've learned this about relationships: love is a choice. If YOU ARE TRULY HAPPY with this guy, then just go buy some tasteful sex tapes for couples, try new things and see if it gets better. The videos teach people how to enjoy intimacy together in a loving, open, new, and safe way. It is possible that his sexual style is just completely different than yours - which can be changed through these tapes:) (link at bottom of page)
If you are not happy in other ways in your relationship this could amplify the lack of passion....how's the day to day stuff with him? Do you find him physically attractive?