Sadly, not all boosters have age minimums on them. And even the ones that do sometimes have unreasonable ages, such as 1 or 3.
This is a very tough situation. One of the first things is to check the law in your state. If he's breaking the law, that might give you some leverage. You might even be able to talk a sympathetic cop into catching him in the act and writing him a ticket (some cops will write tickets requiring a visit with a child passenger safety technician to make the appropriate corrections and sign off). This method is a long shot and probably only has a chance if you either (a) have a cousin who is a cop or (b) live in a very small town. It also would require the laws to be written in such a way that makes it possible for him to be pulled over and ticketed for this. In many states, there are no teeth to the laws, even if laws are otherwise adequate.
The reality is that most parents who do this fall into 1 of 4 categories: (1) - Most commonly, they really don't know the dangers. If it's presented respectfully and carefully they will make the necessary changes. (2) They know the dangers but are having such a hard time affording the proper car seat that when it's a choice between paying the electric bill or getting the child a proper car seat, they gamble that they won't get in an accident rather than risking having the electricity turned off. (3) They know the dangers but they really don't care, perhaps due to depression, a morbid sense that their kids would be better off dead, sociopath tendencies, or spite against an ex-spouse, a parent, or just a system that dares to tell them how to raise their kids. Or (4) They know the risks, but they have such an overgrown immortality complex (sometimes masked as religious "faith" or perhaps confidence in a psychic reading) that they really believe nothing bad can happen to them.
It's important to figure out which category he falls into. If it's (1) or (2) you might be able to get through to him. If it's (3) or (4) you are probably wasting your breath, although you never know. If you can address the root cause, you might have a chance once they realize what they're doing.