Hi.
If you are absolutely clear that you don't want to go down on your boyfriend, just let him know. He will have to decide whether he wishes to continue to engage in oral sex with you or not.
If you're not completely against it, however, and are just afraid, you may wish to address your specific fears with your boyfriend and ask him to help you "wade into" oral sex.
For example, if you're afraid you don't know how to do it, ask him what he likes. Or if you're afraid of choking, ask him not to thrust into your throat for now, to let you control the speed of movement and depth of entry. If you are turned off by the notion of him ejaculating in your mouth or worried about the safety of that, you can ask him to pull out before he ejaculates or ask him to wear a condom during oral sex.
Either way, tell him you would like to delve in at your own pace. Communicate back and forth about what you are comfortable with and what he likes during the process and, in the end, you might both just get what you want. He may feel better about things being more equal and you may overcome your anxiety about oral sex to boot.
Good luck.
