He may have seen it.
Would putting “él” at the beginning of the sentence still be grammatically correct and sound natural? As an English speaker translating in my head that feels more natural, but I’m not sure if it would be break the sentence grammatically in Spanish.
You can put él at the beginning, but it sounds contorted. The English translation isn’t parallel to the Spanish, because ‘puede’ is impersonal.
A parallel English translation would be ‘It might be (puede) that he has seen it’.
Or going the other way, you can say ‘Él puede haberlo visto’ – in this case, él is the subject of puede and so it’s natural to put it at the beginning.