...calls himself Muslim but doesn't even know the shahada and what it is + does not do anything else (prays like once a month/fast for like 3 days in Ramadan)
The determination of one beeing a kafir can only be stated by scholars. This because the act itself, that makes someone leave islam, could happen because of ignorance.
Many of the scholars are from opinion that if one leaves the salat it will make him a kafir. So you should always try to help remind those people of its importance. However, calling one a kafir is not up to us.
If one is not willing to take your advice or even to listen to you, the best way to respond is with sabr.
Our best example for this issue comes from our prophet peace and blessings be upon him. He tried over and over again to help those who were astray like the people from Quraish to lead them on the right path.
But remember, it is Allah subhaanahu wa taala who decides whoes eyes will see, and who will remain blind.
Every effort you will do to help them, will be recorded and you will be rewarded for it. Include them in your prair and ask for their guidance. Many of the ummah are lost from the righteous path.
As beleivers we can only practice sabr and try to guide them - sometimes over and over again...