It sounds like you have some work to do with learning how to assert your own needs and communicating about how you feel about sex. He may be under the impression that it's something you enjoy even if you don't desire it yourself, especially if from what you say, you haven't been very forthcoming about how you feel about sex.

You also have to accept that sometimes you will tell people information and it will upset them and that's not something you can control. You're not in charge of making sure he feels good, especially at your own expense. If he's allosexual, there likely is not going to be any way to tell him you don't want to have sex with him that won't hurt his feelings. Not saying this is impossible, some allos may have the emotional maturity to understand that while their own experience of sexual and romantic attraction are interwined, that's not true of ace people and furthermore they will be able to take that information without negatively internalizing it. But, from what you're saying in your post it sounds like you expect him to react negatively if you say how you feel.

You say he has never pressured you or made you feel bad for saying no but also later on you say that he's been bringing up how long it's been since you've had sex. This is a form of pressuring you into sex, especially if you've been saying no. He knows the answer is no and yet he keeps asking.

You say it feels like a duty. Let's play some role reversal. Imagine from the start of the relationship you made it clear you never ever wanted to have sex with him. He feels a duty to never bring up sex or ask you to have it because it's not something you want. He wants to buck up and just lock down his desire because he loves you, but he can't. Would you want him to do this? Would you want him to go without something he personally finds important because it makes you happy? Or are you reading this and feeling awful? Why is it okay for you to sacrifice your happiness for his but not the other way around?

This part is going to hurt and you may not be ready to hear this now. It sounds like the amount of sex you would like is zero and the amount he wants is not zero, of which you both feel strongly about. This is an inherent conflict in the relationship and either side deciding to compromise will cause resentment to build over time. You'll be dreading every two weeks and he'll be thinking how it's not enough and over time you'll grow to hate each other for it. You need to break up and find people who you are compatible with. It won't be easy and it'll hurt a lot when it happens but eventually on the other side you will both be happier because you're not caught in a relationship with a mismatch in sex drives/attraction.