Tell your boyfriend that you aren't one of his former relationships. If he's endured negative relationship experiences, he may fear being hurt again. His feelings of jealousy and insecurity are compounded particularly if a previous girlfriend cheated.
Persuade him to build on his outlook of self-worth. Explain to him in detail that he is good enough to be your boyfriend. Adding that you find insecurity unappealing is also a good idea.
Speak with him in a comfortable environment about his feelings. Let him know that his jealous feelings are becoming problematic.
Talk to a professional counselor. Ask about payment plans for couple rates. It can sometimes take the insight of a professional to help your boyfriend work through his feelings of low self-worth.