You know, I can't honestly say. But what I'd suggest is that what you might think and talk about is less about asking for this specific way for him to express his love for you and more about ways to meet more in the middle with this, where you can get some of what you need to feel more loved and more affirmed in that love -- which is, what I assume, what this is really all about -- and he can find some ways to do that that feel like authentic expressions for him, and thus, aren't going to feel forced or like a drag or fake. Know what I mean?
I mean, you want real expressions anyway, so if you wind up with him saying it to you more *because* you ask, it might wind up often feeling that the kind of apology we get from someone who someone else told to say "Im sorry." Not great.
So, maybe you can first have a think yourself, by yourself, and then initiate a talk with him where you talk about how you feel like you need to know he loves you in some concrete ways more than you often feel like you do. You can talk about how, for you, that can be saying "I love you," and maybe that's one way he can do it, but then you can talk about, hopefully together, some other ways, and perhaps come up with some things you both feel good about as expressions of that love. Like I said, I don't know what that would be, it might be spending time with you, or planning special dates, giving gifts, making something for you (dinner, writing a song, painting a room for you), physical affection, doing things you like to do...there's a world of ways we can express love for one another and there's no one right one, even if there might be a couple we're used to, or feel like are somehow better than others.
And that's really my last piece of advice: it might help to see if you can't let go of some of your attachment to this one way of expressing that a little. I get it, I get it's important to you. (Me too.) But I also know that I've been with people who could say it easy and actually didn't even love me that much, or weren't all that good at loving me, whereas some other folks who just weren't big with it have been really great at loving me and just showed me in other ways. And of course, there have been folks who didn't love me well and didn't say it. Point is, I think what's most important is how well someone actually loves us, and if THAT is the problem...well, that's a whole other conversation, and saying "I love you" won't fix that. But if someone does actually love you and treats you with love and just has other ways of expressing it than the given way you're used to or want? They're probably worth learning to flex a little. <3
Never doubt that a small group of thoughtful, committed citizens can change the world. Indeed, it is the only thing that ever has. - Margaret Mead