Any child of Muslim parents is considered a Muslim, and Islamic law contains precise and detailed provisions regarding children. Islam is the system of beliefs, rituals, and practices traced back to the Prophet Muhammad (c. 570–632 C.E.), who reportedly started his mission in Arabia in 610 C.E. Islamic law is contained in the Muslim holy book, the Qur'an (or Koran), as revealed to Muhammad.
Islam - Islamic Law, A Childs Upbringing, Duties of Parents, Religious Education - Encyclopedia of Children and Childhood in History and Society
I fail to see why this is so confusing. This isn't a question of whether its right or not based on your moral judgement, its based on the fundamental law of Islam. He was born to a Muslim father and given and Muslim name. According to that faith, he is a born Muslim. It doesn't matter how often or not he practiced the religion as a child.
Later he decided to be baptized as a Christian so that is now his faith.
Those are the facts.
I fail to see what you are accomplishing by arguing against how a religion views a child.