My son has been well catechized. He knows the truth. He refuses to engage in a rational discussion about it.
Assuming he has been properly catechized (and I DO believe you on this), this is a very tough one to speak on. Here are my opinions (which are free, and you get what you pay for).
I am assuming he has been confirmed, which means that he is ultimately responsible for his own formation at this point. However, as parents, we are still obligated to instruct our children in the faith. To not do so is mortal sin. He is also not a legal adult yet, and is very much a child, although I have no doubt that he feels quite differently about his stage of mental and emotional development.
My oldest daughter is 14, so I have great empathy for your dilemma. Were my daughter to come to me with this here is what I would do…
I would reemphasize why she should not leave the Catholic Church (fulness of Truth, Church Jesus established, apostolic succession, valid sacraments - including the Eucharist, mortal sin, etc. etc. etc.) I would then explain to her that she is still under my direct authority and that she would still be attending Mass every week because not only is it a mortal sin for her not to, but it is also a mortal sin for ME not to make her.
Participation in Mass,including Holy Communion, is completely at her discretion.
I MAY be open to the possibility of her attending Protestant services, but it would not replace her weekly obligation, and I would be attending with her. She would also be made very aware before hand that she would not be participating in communion services or any other things that may lend to the appearance of scandal. (This may have the added benefit of opening the door to the rational discussion your son refuses to have. The sermon may be “good”, but could also be completely WRONG. If they have a communion service, it would be a good springboard for why it falls so woefully short of the actual Body, Blood, Soul and Divinity of the Eucharist.)
Of course, my daughter will gladly give testimony to the fact that I am the meanest father alive, and only you know your son and how best to deal with him.