Personally, I think it's crazy that children anywhere are *required* to use boosters as late as age 11. If you think it makes riding in a particular car safer, then certainly follow your parental instincts. But I know that - despite changing laws and guidelines - keeping our youngest in a booster past kindergarten will be a hard sell with my entire family, since his older brothers (ages 13-17) either weren't required to use them at all, or were out of them (legally) shortly after preschool.
So, it should be clear that my ideas below are not based on feeling horrified at the idea of a child younger than 11 being without a booster.
It sounds problematic for you to let your kids stop using boosters - under any circumstances - earlier than you've told your ex he may do so. Your assessment miight be spot-on (that you can tell which cars don't need one and your ex can't). But you can't expect an ex-spouse to accept that kind of reasoning. I'm sure there are things he thinks he does/understands better than you do, yet if he insisted you do something with the kids "his way" - yet eschewed those rules in his own home - surely you'd find that offensive.
If you get along with your ex well enough that, currently, he's listening to you about the boosters, then that kind of cooperation and respect is a good thing to maintain - for you and your kids. Even assuming you're right about everything you said, in your shoes I wouldn't be willing to sacrifice the functional dynamic with my ex, to let my kid out of boosters earlier than the age I set for my ex. Next time you tell him something is important for the kids, you want him to trust your judgment, not say, "Yeah, whatever. She just likes to think she can control my parenting time. She doesn't even do what she tells me to do!"