Men grow up with this big burden of assumed sexual knowledge, as if a copy of the "Kama Sutra" (horrible sex advice book, by the way) must grow alongside them in utero so they emerge ready to satisfy and blow away all their lovers. Why do you think the "performance anxiety" trope is exclusively related to men? Show me one clip of a major film in the last couple of decades featuring a joke about performance anxiety in a sexual setting that refers to a woman instead of a man and I will pay you $100. ( I'm not kidding. You can submit the link through the "Ask A Question" form at the bottom.)
Your partner is probably dealing with discomfort that his role as a man is being undermined and if he does something wrong you will know and you will judge him with your judging eyes of judgment. These are his insecurities. Sure, you can talk to him about it. You can reassure him that you would love nothing more than to explore the sexual seas with him and to teach him everything you like and learn everything he likes. You can point out that because you’ve had experience with other people, you know what you want and how to tell him. You can even wax poetic about how every person is different so it doesn’t matter how many people he’s slept with, he’ll always have to learn the next person’s preferences!
But, in the end, these are his insecurities. He will choose to either believe you or to cling to that emotional raft that is his fear of sexual inadequacy. You’re not responsible for making him feel better about this or showing him the light. If none of the conversations suggested above ease his discomfort, you may want to find a lover who would never bother asking how you many people you’ve slept with because they know it’s irrelevant. Do not be afraid to break up with him if he continues to make this an issue.