I recommend you scrap the OpenDNS and just tell him to stop using porn. There is no technical solution that he cannot work around if he tries hard enough. All you are doing is turning this into a game for him. Instead you need to negotiate a solution. The default should be no internet access at all for anyone. So that might encourage him to get going. Have you read up on the Policy of Joint Agreement?

Of course, once you have his agreement, without his knowledge, you will check that he is keeping to the agreement. You can quietly set your router to log the websites he visits, that is the simplest and easiest although it will not stop him using his phone or apps to chat. Better to put spyware on his computer. Don't forget to put it on his phone too. But if he knows you are doing this it will become a game again so be absolutely sure he cannot detect it. Test it by running anti virus software after it is installed.