Based on our conversation so far, I will advise not to call authorities on him. Rather try to solve the issue by involving all siblings & his mother. Family members who are close to him shall share their concerns and request him to be more accommodating for the sake of his mother and for the sake of other family members. Severing the ties will not only hurt him more but may also hurt his mother ( after all she is the mother and can't see his own child suffering). Family members should keep on visiting them regardless of how he behaves. It is best to keep the ties of kinship even when this person is hard to deal with. I know it can be hard but don't give up, keep visiting him and his mother. Regarding keeping the ties of Kinship, I want to quote few hadiths and hope these will help.
Narrated `Abdullah bin `Amr:
The Prophet (ﷺ) said, "Al-Wasil is not the one who recompenses the good done to him by his relatives, but Al-Wasil is the one who keeps good relations with those relatives who had severed the bond of kinship with him."
حَدَّثَنَا مُحَمَّدُ بْنُ كَثِيرٍ، أَخْبَرَنَا سُفْيَانُ، عَنِ الأَعْمَشِ، وَالْحَسَنِ بْنِ عَمْرٍو، وَفِطْرٍ، عَنْ مُجَاهِدٍ، عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو ـ وَقَالَ سُفْيَانُ لَمْ يَرْفَعْهُ الأَعْمَشُ إِلَى النَّبِيِّ صلى الله عليه وسلم وَرَفَعَهُ حَسَنٌ وَفِطْرٌ ـ عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " لَيْسَ الْوَاصِلُ بِالْمُكَافِئِ، وَلَكِنِ الْوَاصِلُ الَّذِي إِذَا قَطَعَتْ رَحِمُهُ وَصَلَهَا ".
Reference:
Sahih al-Bukhari 5991
In-book reference: Book 78, Hadith 22
USC-MSA web (English) reference: Vol. 8, Book 73, Hadith 20
https://sunnah.com/bukhari/78/22
Abu Huraira reported that a person said:
Allah's Messenger, I have relatives with whom I try, to have close relationship, but they sever (this relation). I treat them well, but they treat me ill. I am sweet to them but they are harsh towards me. Upon this he (the Holy Prophet) said: If it is so as you say, then you in fact throw hot ashes (upon their faces) and there would always remain with you on behalf of Allah (an Angel to support you) who would keep you dominant over them so long as you adhere to this (path of righteousness).
حَدَّثَنِي مُحَمَّدُ بْنُ الْمُثَنَّى، وَمُحَمَّدُ بْنُ بَشَّارٍ، - وَاللَّفْظُ لاِبْنِ الْمُثَنَّى - قَالاَ حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، قَالَ سَمِعْتُ الْعَلاَءَ بْنَ عَبْدِ الرَّحْمَنِ، يُحَدِّثُ عَنْ أَبِيهِ، عَنْ أَبِي، هُرَيْرَةَ أَنَّ رَجُلاً، قَالَ يَا رَسُولَ اللَّهِ إِنَّ لِي قَرَابَةً أَصِلُهُمْ وَيَقْطَعُونِي وَأُحْسِنُ إِلَيْهِمْ وَيُسِيئُونَ إِلَىَّ وَأَحْلُمُ عَنْهُمْ وَيَجْهَلُونَ عَلَىَّ . فَقَالَ " لَئِنْ كُنْتَ كَمَا قُلْتَ فَكَأَنَّمَا تُسِفُّهُمُ الْمَلَّ وَلاَ يَزَالُ مَعَكَ مِنَ اللَّهِ ظَهِيرٌ عَلَيْهِمْ مَا دُمْتَ عَلَى ذَلِكَ " .
Reference:
Sahih Muslim 2558 a
In-book reference: Book 45, Hadith 25
USC-MSA web (English) reference: Book 32, Hadith 6204
https://sunnah.com/muslim/45/25
Invite him to pray, socialize with him and spend more time with him.
Try to convince him for counseling to fix his mental issues. It is quite possible that this individual may have deep psychological issues and social anxiety, therefore, seeking a professionals help is important.
Advise him to find an activity and goal, such as walking 3-5 miles every day. This will not only help him to have a positive mind set but will also provide other health related benefits.
Try to reach out to his kids and advise them to visit him more often ( this is the time when he needs them the most).
Make dua for him & remind yourself and others that we are doing this to please Allah SWT.
Finally, please have a look at the article below regarding Islamic law for inheritance. I know this was not your main concern or question but it came up during the conversation. I think its important that the inheritance be divided based on Islamic laws so that those who have left the inheritance may not be asked about it on the day of Judgement.
https://islamqa.info/en/answers/307/rules-of-erth-inheritance
And Allah knows the best!
May Allah help this brother, his mother and other family members and help them to keep the ties of kinship even in hard times. Ameen!
Ma'aSalaam