Pled is a marginal word. There are arguments both for and against its use as past participle.
Plea is a NOUN.
Plead is the verb that can be conjugated.
A related word, etymologically, is placate. One would not say "He plaed" for "he placated," and the argument against pled is strong if you rely on the etymological root.
Phonically, the argument against pled becomes more apparent. Compare:
https://www.theconjugator.com/englis.../to+plotz.html
https://www.theconjugator.com/english/verb/to+plod.html
https://www.theconjugator.com/englis.../to+plead.html
Which comes closer to:
https://www.theconjugator.com/english/verb/to+ply.html
Plea and ply do not have a trailing consonant, which thus makes for the counter-argument in favor of pled, but since plea is a noun and not a verb, the argument is questionable at best.
I slightly favor "he pleaded," but recognize "pled" as a vulgar term in common use.