I agree with everything said here and was thinking that Dog's Lady has a good point about 69. You said you have never been into giving or receiving oral. Maybe since he wants you to go down on him, you could start by having him go down on you and see how you like it. If you can relax into the feelings his tongue gives you and it is pleasurable then that may help with your thoughts about it. I am in a lesbian relationship but I was married for 12 years and did give oral sex to my husband. Everyone is right about it not being gross or disgusting as long as everything is clean. It is simply a part of your Master's body to love.
On another note, I find that if my Master (she prefers that title) wants me to do something that I have reservations about then we talk about and I think. I think of what submission means to me and how much I love her and want to meet her every desire. Sometimes, submission is not always easy. I usually end up deciding to at least try to do what she is asking. If she has enough respect for me to "ask" rather than demand, then I feel that I want to respond in kind by trying, if for no other reason than my submissive commitment to her. When I think about this, it helps put things in perspective for me. I hope this helps. But like everyone here is saying, baby steps are important. Size and taste can be major factors at play in giving oral sex to a man. There are no hard and fast rules that say you have to do it a certain way or that you have to take all of him in your mouth...nothing like that. IMO it is more about pleasing him and that can often be accomplished in many ways. Personally, the feel of a tongue on me down there is so pleasurable that simply licking can be all I need. If he sees that you are willing to try, that may be a big turn on for him and a great way to show your willingness to please him and to submit.
Good luck and remember to take it slow.