Maybe he is giving 100 percent but it doesn't match up with what your idea of his best should be. I would bet he may be....
One: Tired. If his league practices in the late afternoons like kids' leagues do here in football, he might be physically tired by the time practices roll around. Kids here are practicing from 5-7 p.m. in August heat. I don't know why they don't collapse.
Or, two: Pressured to perform a certain way in a sport he likes. Whether he can say it or not, he is fully aware that you are disappointed in him, even if you never say so. Kids his age pick up on everything, every small frown, crossed arms, lots of questions about "Are you doing your best?" Instead of spurring him to "hustle back to the huddle" that pressure may be causing him to be more cautious and slower. Logical reaction? No, but he's seven. And mom (and dad?) are there watching his practices every time. Can you drop him off and leave and just pick him up saying "Did you have fun?" not "Did you do your best?"
Or, three: Pressured to be in a sport he doesn't really click with. Maybe despite what he says, he doesn't really like football well enough to hustle for you -- but if it's the only sport or other activity he's tried so far, of course he will tell you "I love it, don't take me out of it."
Kids strongly identify themselves by their activities: "I play football," I am a ballerina," "I like to draw stuff and do cartoons." He is already labeling himself from last year: "I play football, it's what I do," even if the label is no longer fitting himself. Football is what he does and he knows it's what his parents want him to do.
I'd ratchet the questions way down, and even stop staying for practices so you are not on the sidelines at all watching him. Let him play some more with no talk about how he's doing and probably finish the season unless he's miserable later. Yeah, you want him to stick out a commitment, but you don't want to make new commitments to something his heart isn't into.
Then when football season is over, try him with something totally new -- soccer, fencing lessons (yes, they do them as young as seven and it's a sport, not just waving sticks around), a martial art, drawing/painting/clay lessons, gymnastics, Cub Scouts, yeah, even dance if he wants, boys do dance...it doesn't have to be all sports or even any sports.
I'd back off, be sure his dad backs off too, and direct him to something else outside football season so he can test boundaries. He does not have to play football now or later just because he used to, and he (and you and your husband) are NOT "quitters" if he declines to play after this season. He's a child, not a professional football prospect in his senior year of high school. He needs exposure to other things. In some places other sports parents frown on parents whose kids "aren't on the team this year." Don't let that kind of silliness get to you. You as parents, and he as a kid, should not feel pressure to be "back on the team" because it's what he did earlier.
By the way, the coach saying "Where did my player from last year go?" OK, that's pressure. Gently put, but it also put your son on notice: "You're not as good as you were" is the subtext of that. If this league is for fun, what does it matter if your son is not hustling and playing like last year, unless the coach has his eye on trophies?