If I signed one lease by myself and later signed another lease with my son - are they both valid under D.C. law?

If they were both with the same landlord... the second one would have been a mutually agreed modification to the original. And the original would be invalidated at that time.

You can't have two different valid leases on the same property.

If the landlord accepted and signed both the one with the later date is the only valid one.