Since this person is a Catholic, C & E only, I think I would try to engage him in a conversation about what the Mass is, why it is perfect worship, and why the Father seeks those who worship Him “in Spirit and Truth” (Jn 4:23-24) and why the Holy Eucharist is “the source and summit of the Christian life.” (Catechism #1324, quoting Vatican II’s Lumen Gentium #11).
The Catechism affirms this:
1407 The Eucharist is the heart and the summit of the Church’s life, for in it Christ associates his Church and all her members with his sacrifice of praise and thanksgiving offered once for all on the cross to his Father; by this sacrifice he pours out the graces of salvation on his Body which is the Church.
Does he realize this? Does he realize what he is neglecting? What he is missing? Why would he miss such holy treasure?