“He also argued that being born on the soil made someone a “citizen”, and therefore all slaves born in America were free.”
Do you have a link to his brief where he made that argument?
You know the court records are lost. You must know that if you've looked at the case as well as you seem to have done.
You have to read his history, and then surmise he did the same thing he always advocated doing.
You can also get some insight by reading what his co-counsels wrote.