I recently found out that my long term boyfriend was born with only one testicle and as yet we have no sexual relationship. How do I encourage my boyfriend to have a normal sexual relationship with me and make him see that only having one testicle doesn't make you not a real man.

Has he told you this is why he has no confidence, or is this your own conclusion? He may have other reasons he hasn't started a sexual relationship with you (fear of pregnancy, religious reasons, wanting to remain a virgin until marriage, shyness, lack of knowledge, etc.).