You can't really force someone to have sex with you--even in a relationship. Since the basis of a good relationship in good communication talk to him about the problem. Try to be very calm and rational without being insulting. Listen and ask questions to try to figure out his side.
Men tend to be visual. Try wearing clothes that are enticing to him, doing your make up the way he likes, etc... Perhaps this will arouse him and make him want to have sex.
Still, he just may not have as fast of a libido as you do. Everyone's is different. You stated that it is hurting your relationship. This may mean that his being less interested in sex than you are is a deal breaker and this is not the relationship for you.
