2019 filed as HOH with a dependent adult son. I received 1200 for the first round. Should have been additional 500?

You said your son is an adult?  No.   You only get stimulus money for a dependent child under the age of 17.  If your son is an adult you get the $500 credit for other dependents, and that is all.

CREDIT FOR OTHER DEPENDENTS

https://ttlc.intuit.com/questions/4499708-what-is-the-500-credit-for-other-dependents-family-tax-credit