I think you should tell him your feelings. If you deny something that feels to be part of your nature, you may build resentment or end up revealing your nature down the road. So I think it is better to let him know how you are feeling.
Maybe you could try a limited monogamy. Tell him you will try it for a year with him and see if he agrees. During that time, he may come to accept polyamory or you will find out you could be monogamous.
I think ethically, if you are having doubts, you need to let him know. Let him make an informed decision to be with you. I think that will help long term nomatter what you choose.