Please seriousonly, this is serious. Just one of his eye lids turned purple, without being hit by something or someone, just by itself. What can this be a sign of? Stress?
Thanks

it is a sign that he should see a medical doctor.

There r a few reasons this could happen:
A disorder of the blood where it fails to clot normally - that would also include bleeding easily & not stopping bleeding with an injury - & it can also happen with anticoagulant medications. Is he taking anything to prevent blood clotting?

Bacterial infection is another

And u say he was not hit - can not help but hope it is this - which could have happened without his knowing, perhaps in his sleep

I'd call a family physician & check with them