If they don't need the sleep there isn't much you can do to "make" them sleep other than giving them drugs to knock them out.
Yes, kids need a lot of sleep. Most don't get it.
I think it is very cruel to put a kid in bed when everyone else in the neighborhood is outside playing, running around having fun. It is silly to put kids to bed when the sun is still up outside.
Let him stay up, it's summer. Once school starts the days are shorter and it's easier to get them to bed and up on time. He may even fall asleep in the car on some days if you are out running errands. But he will most likely not be sleeping as much as he was at the end of Spring.
9pm to 7am is 10 hours so if he is in bed around 9pm his is getting enough.