How do I stop my boyfriend from feeling insecure about his Size

He’s 6 inches!!!! That’s like average size! I don’t even know if that’s the size I don’t like to ask him because I don’t really care. I can’t ever seem to make him cheer up. He won’t ever stop joking about it or saying he never pleases me because of it. And it is true I don’t climax but I don’t even know the difference in size. I feel like if he was bigger it would hurt and it wouldn’t be enjoyable. I’ve never been with a big size anyways and even if size mattered how do I cheer him up about it. It’s so annoying.