When someone very close to us is angry with us, they know us well enough to know what accusations will hurt us most. Whether they are true or not is quite incidental.
Keepingquiet your son will know how important truth is to you and that being accused of lying, will cut you to the quick. He will also know deep inside, even though he will not admit it, that his own actions have brought him to this impasse and he will be defensive and denying everything.
You have all my sympathy in his sad situation, but you know what your son says is wrong, just remember he said it because, sadly, he wanted to get you where it would hurt most and this is what he did. It doesn't mean that what he said is true.