Yes you should disown him. Atheism will lead him to hell and if you love him then you shouldn't let that happen.

Whoever spares the rod hates his son, but he who loves him is diligent to discipline him. Proverbs 13:24

Some people say you should accept your children regardless of what they do, but if you do that then you are showing god that you hate them. Do everything in your power to get him back on the right path, if he won't change his mind then accept that he will go to hell but you have done everything you possibly could and God will see that and you will be blameless. As Christians all we can do is give people the information they need to accept god, in the end the decision is on that person and we have done everything in our power to try and save them.

[I think that the person who asked the question was a Poe, but I think this guy who answered it is serious]