Hard for me to imagine "On jest?", but if you substitute it with a name, sure. But then it could work for "Is Marek here?", potentially "Is Marek there?"... I don't know if we could consider it a translation.
How a sentence can be just "He is"? It could be "There is a man"or something like this. To be needa always an atribute except when it is an answer to a question...
if u were know rus, u would understand it quickly and easier. "Он есть" "Он существует" and "On jest" = "Он есть" , transletaed "He is", "There is a man" <- false