All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
You have mentioned some evil matters about your brother, among which is him abandoning the prayer with the exception of the Friday prayer, being disobedient to his parents, having bad manners, and insulting you with those ugly words.
He is indeed upon very serious danger unless Allah has mercy upon him.
The most important thing that we advise you to do is to be patient with him and to supplicate Allah to guide him and enable him to regain his senses.
We stress the importance of the supplication of the parents for him, as it is answered. Abu Hurayrah, may Allah be pleased with him, narrated that the Prophet said, "There are three supplications that will undoubtedly be answered (by Allah): the supplication of a victim of injustice, the supplication of a traveler, and the supplication of a parent for his child." [Ibn Maajah]
You must advise him in a kind and gentle manner, out of compassion for him, hoping that he would become righteous and return to his Lord.
If this happens, praise be to Allah; otherwise, there is nothing wrong with deserting him if a valid benefit is expected from doing so. Otherwise, it would be better not to desert him.
If a person calls another Dajjaal, Fir‘awn, or Abu Jahl, then there is no doubt that he has committed a clear sin. If you are asking whether this is included in declaring one’s fellow Muslim a Kaafir (disbeliever), then the answer is that this is not necessarily so. Perhaps he only meant drawing a resemblance in having bad traits.
In addition to this, some scholars interpreted the hadeeth about the threat for someone who says to his brother, ‘O Kaafir’, to mean minor Kufr.
Shaykh Ibn Taymiyyah said in Al-Istiqaamah, “The Prophet called him a 'brother' when he said so, and he said that it will fall on him it if it does not apply to the other person. If either of them goes completely out of the fold of Islam, he would not be considered as 'his brother' [i.e. the Prophet would not have said 'his brother'].”
Besides, Ibn Qudaamah said in Al-Mughni, “These ahaadeeth are meant to strongly deter (someone) and resemble him to the Kaafirs, but they do not mean that he really is a Kaafir.”
Allah knows best.