He fasts in Ramadaan then he does not pray after Ramadaan
Question

If a man is keen to fast in Ramadaan and to pray in Ramadaan only, but he gives up praying as soon as Ramadaan is over, is his fasting valid?
Praise be to Allah.Prayer is one of the pillars of Islam, and it is the most important pillar after the Shahaadatayn (twin testimony of faith). It is also an individual obligation. Whoever does not do it because he denies that it is obligatory or he does not do it because he is lazy and heedless, is a kaafir. As for those who fast in Ramadaan and pray only in Ramadaan, they are trying to deceive Allaah. How bad are those who only acknowledge Allaah in Ramadaan. Their fast is not valid if they only pray in Ramadaan, rather by doing this they are kaafirs who are guilty of major kufr, even if they do not deny that prayer is obligatory; this is according to the more correct of the two scholarly views. The Prophet (peace and blessings of Allaah be upon him) said, “The covenant that stands between us and them is prayer; whoever does not pray is a kaafir.” Narrated by Imam Ahmad, 22428; al-Tirmidhi, 2621; al-Nasaa’i, 431; Ibn Maajah, 1079 – with a saheeh isnaad from Buraydah al-Aslami (may Allaah be pleased with him). And the Prophet (peace and blessings of Allaah be upon him) said: “The head of the matter is Islam, its foundation (or its pillar) is prayer and its pinnacle is jihad for the sake of Allaah.” Narrated by Imam al-Tirmidhi, 2616, with a saheeh isnaad from Mu’aadh ibn Jabal (may Allaah be pleased with him). And the Prophet (peace and blessings of Allaah be upon him) said: “Between a man and kufr and shirk there stands his giving up prayer.” Narrated by Imam Muslim in his Saheeh, 82, from Jaabir ibn ‘Abd-Allaah al-Ansaari (may Allaah be pleased with him). And there are many similar ahaadeeth.
And Allaah is the Source of strength. May Allaah send blessings and peace upon our Prophet Muhammad and his family and companions.