Uncomfortable about doing anal.

Hello ladies,
This topic is a little different for me to talk about, but here it is. My boyfriend and I have been together quite a while. I lost my virginity to him and we have sex relatively often. For a while now, my boyfriend has been talking to me about trying anal because he has never done anal with anyone and he's "intrigued" by it, I guess. Anyway, I'm not comfortable doing anal and I make that known to him, but sometimes when he is feeling like trying it out, I tell him, "stop babe, I'm not okay with this" or "honey, don't, I'm not comfortable" but sometimes he'll continue to try and slip it in anyway and it makes me feel so sick inside simply because I don't think I should ever feel uncomfortable in any sexual situation and my significant other should respect that, right..? Please give me your guys' advice on how to handle this situation and bring it up to my boyfriend.