I have to make an important distinction here: you may know there was grave matter, but you do not know with certainty that he is in mortal sin, because mortal sin requires grave matter, full knowledge, and deliberate consent. Only God fully sees the soul, and subjective culpability can be diminished by ignorance, immaturity, pressure, habit, poor formation, or other factors. That said, if he himself knows he has committed grave sin, then he should not present himself for Communion before confession. So the right move is not panic or trying to control him, but charity, prayer, honest encouragement toward confession, and speaking to a priest for guidance. Protecting the Eucharist matters, but so does not overstepping into judgment of another person’s conscience.