My son is 20vand on his last year of high school how do I qualified for that credit of him being still in school

Per IRS: Expenses paid to attend a private high school don't qualify for an education credit because a high school isn't an eligible educational institution.

If you are asking about the $500 Credit for Other Dependents, the credit allows taxpayers a credit for certain dependents that don't qualify for the Child Tax Credit, such as qualifying children age 17 or older, adult dependents, and dependents who have an Individual Taxpayer Identification Number (ITIN).

To be eligible for this credit, your son must fit the definition of a qualifying child or a qualifying relative, as defined here:

Qualifying child

Qualifying relative

Q. How do I qualified for that credit of him being still in school?

A. You just claim him as a dependent, to get the $500 "other dependent credit". Just enter his info, at the dependent screens, under personal info. 

There are two types of dependents, "Qualifying Children"(QC) and Other ("Qualifying Relative" [QR] in IRS parlance even though they don't have to actually be related). There is no income limit for a QC but there is an age limit, student status, a relationship test and residence test.

Being in High school allows him to continue being a QC, which means ther is no income limit to him being your dependent