Insecurities your boyfriend feels may show up in arguments, extra jealousy, moodiness or a break up. Insecurity within anyone can have many causes, but it is important to remember that in most, if not all, cases the insecurity began long before you entered your boyfriend's life. Certain attitudes and behaviors on your part can sometimes help the relationship, however, to the benefit of both you and your boyfriend.
Discuss his insecurities in a non-accusatory way. In other words, don't start out by angrily saying, "Why are you so insecure?" Instead, tell him you can sense he is insecure about some things and that you'd like to help him feel better in the relationship and about things in general.
Let him know he can communicate with you if he has jealous thoughts or feelings to put his mind at ease. It is tempting to become angry or annoyed when a partner is jealous, but try to remain calm. The exception to this is if he becomes verbally abusive about it and accuses you even after you've told him the truth and tried to alleviate his concerns. No one should have to put up with verbal abuse stemming from her boyfriend's insecurity.
Gently suggest that he explore where some of the insecure feelings really stem from. Insecurity does not just happen overnight and much of it is probably due to other things that have happened in his past. If he can pinpoint the cause of some insecurities, you can help him through reassurance. For example, if his insecurity is due to people in his past teasing him about his looks, spend extra time reassuring him that he is handsome. If he is worried that you may cheat because former girlfriends have done so, remind him that you are different and reassure him that he is the only one for you.
Suggest relationship counseling for both of you to deal with the issues. Many therapists have experience in this kind of counseling and may be able to help you both deal with and resolve his insecurities in a healthy way.