Can an ex revoke a court order about child support after 10 years?

Only the court can revoke a court order.

I have 2 boys 1 will be 18 in nov. and the 2nd one will be 18 in 2 year does my child support decrease after the 1st boy turns 18

Depends on where this is and what the actual order specifies.

Some states it ends at 18, but in someStates its longer and may include college too. The most important thing is what exactly does the current support order specify.

An ex can challenge or ask for a modification of a support order anytime, Just as the other party can ask for more money every few years if salary has increased for ex.

If child support is just to 18, and not to 21 or children in college or what ever your order says. Then if support ends at 18, yes the payment goes down, since it is for only one child.