I have already disvowed the domain. Still he is creating more than 100k links daily.
How should I report this to Google?
With so many links, you have no choice but to disavow them using the Google Disavow Links Tool. You will need a Google Search Console (webmaster tools) account to do this. You can get started using this page from Google: https://support.google.com/webmasters/answer/2648487?hl=en
This should be a simple enough task and not too scary. You should be able to disavow links using the spammers domain name without having to specify all of the links.
Short of that, there is nothing you can do to let Google know directly that you are not pleased.
Indirectly, you can use your .htaccess or other similar method to break any referral from this site. In other words, if someone were to follow the link from the offending site, then they would get a 410, 404, or some other appropriate error. If someone were to follow a similar link from any other site, they would get the page. This technique has been used in the past, however, because it is rarely used mostly due to lack of knowledge, no-one can really say what the effect is. The message is fairly clear. Whether Google takes the hint is another story.