Why am I not seeing other dependents tax credit $500? My son meets all 4 tests: he is 21, student, and does not pay for his own support?

[Hal_Al]

The Other Dependent Credit (ODC) is non refundable (you must have a tax liability to offset).  It is a maximum of $500 and can be less, if your regular tax liability (line 18 of form 1040)  is less than $500. 

Note that the ODC cannot be used to reduce self employment tax, early withdrawal penalties, and some other Schedule 2 taxes, on line 23 of form 1040. 

[BillM223]

Hal_Al's post above is quite correct, and may explain your issue.

Please note that there are a few more requirements to take the Other Dependent Tax Credit than listed above. Please read the IRS page "Understanding the Credit for Other Dependents". You may find it helpful to try the IRS Interactive Tax Assistant to see if you qualify for this credit.