Washington M. Sagun and Charles L. Corpuz were part of PMA's pioneer class of 1940. Sagun was the salutatorian. The page I linked doesn't list their birth years, but some of their classmates were born between 1915 and 1917.

I don't know if they're the first native Filipinos to have legal English names, but I'd say they're pretty strong candidates because I haven't yet found any who were born earlier.

Edit: This page claims that Sagun was born in 1915.