It seems you are following the ethical course, and this is commendable. As you may already know, in ethics we distinguish between ordinary and extraordinary (or disproportionate) care. But while we may licitly refuse disproportionate care, at least in many cases, we cannot intentionally cause the death of anyone (ourselves or another; the "self-ownership" thesis is likely responsible for the view that we can licitly do anything to ourselves as if we were some kind of property, but we are not objects, and what is morally good is objective and not fully accounted for by consent, or utility). We may not purposefully speed up anyone's death, but we can, in some cases, where proportionality is preserved, permit treatment or refuse treatment, knowing that an unintended side effect is the hastening of death (like, perhaps, alleviation of pain).

In any case, know that such circumstances as yours, if we respond to them rightly, perfect us in the virtue of charity, the highest and greatest of virtues.

I wish your son, you, and your family comfort and joy. Do not despair. Fear not. Pax vobiscum.