How do I show him this?

Say you enjoy it. Is he not believing you or something? Does he think you're a liar or a faker? Or are you generally not a person who can express enjoyment? Basically, some people have these problems. I think if you told him that he'd understand, it can actually be endearing I think. It's not exactly something I'd call a flaw, being inexpressive is in some situations in life (if not romantic ones) an advantage.

During sex, say something, anything; moan, scream, wtv. Shout out random things about his performance or his member during sex

Mention it in the morning or sometime during the day that you can't wait until the next one, etc. Just tell him how great it is!
Idk wht kind of dude ur boyfriend is and I've never met a girl who doesn't enjoy sex.

Grab him a little harder, rub his back, moan, breathe louder(if you don't feel comfortable moaning), or, after your done, cuddle up and tell him 'that was great'. It's actually a lot more easy than you'd think. I take it as you're not so comfortable with your partner? Or how long have you been sexually active with him?