Whoever does not fast because he denies that it is obligatory is a kaafir according to scholarly consensus. If a person does not fast because he is lazy or heedless, some scholars say that that makes him a kaafir, although the correct view is that it does not, but he is in serious danger because he has forsaken one of the pillars of Islam on which there is consensus that it is obligatory. And he deserves to be punished and disciplined by the ruler in such a manner as to deter him from this. He must also make up the fasts that he has missed and repent to Allaah.
And Allaah knows best.
See Fataawa al-Lajnah, 10/143.