All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
What has been established with certainty can only be denied with certainty. Since Islam was established with certainty, none has the power to judge that a Muslim is a disbeliever/Kaafir, except with certainty. In principle, your brother is a Muslim; therefore, he can`t be judged as a disbeliever based on mere conjecture, and you should beware that this is a very serious issue, as stated by the learned men of the faith. Al-Imam Al-Ghazali(May Allah be pleased with him) stated: "The mistake of leaving a thousand disbelievers free is lesser than shedding the blood of one Muslim.{Al-Iktisad Fi Al-E`etikad/135}.
Moreover, Ibn Daqeeq stated: "{ If somebody accuses another of disbelief or calls him the enemy of Allah, such an accusation will revert to him (the accuser) if the accused is innocent }.This Hadith is a strong warning to those who accuse Muslims of disbelief. The right thing is that none of the Muslims may be called a disbeliever unless he denies indisputably established rulings of Islam which are known to every Muslim."{Ihkam Al-Ihkam Shareh Umdat al-Ihkam, 2/210}.
In conclusion, in principle, the aforementioned person is a Muslim and remains as such unless proven otherwise by a verdict from an Islamic court.