Your 18 year old does not get stimulus for himself. Dependents do not get stimulus money.
If he had a job and has income to enter on a tax return he can file to get a refund of tax withheld. But he must say on his own tax return that he can be claimed as someone else's dependent. If he says he cannot be claimed --- then when YOU try to file your tax return, your own return will be rejected and you will have to file by mail.
He did work in 2018 and filed a return this past year (2019) and got a few dollars back (didn't work that much). He is a dependent of ours. So because he is 18, in high school, one of our dependents, we do not get stimulus for him?
That is correct. You do not get stimulus money for a child who is older than 16. The stimulus money follows the same rule as child tax credit. You lose the child tax credit in the tax year that a child turns 17 and you do not get stimulus money for a child who was over 16 in 2020. And since he is your dependent he does not get stimulus money himself.