What is the best way to invite an 18-year-old Protestant young man to Islam online, taking into account his adherence to his religion?

The best way to invite this man and anyone else is by that which is best. Allah says: (And do not argue with the People of the Scripture except with that which is best) and (Invite to the way of your Lord with wisdom and good instruction, and argue with them in a way that is best).

Among the methods that can be followed: - Addressing him by appealing to the innate disposition (fitra) which necessitates the Oneness of Allah, and how the belief in the Trinity contradicts fitra and reason. - Clarifying the stature of Jesus (peace be upon him) and his mother among Muslims, and that he is Allah’s servant and messenger, and His word which He bestowed upon Mary and a spirit from Him. - Clarifying that Allah's religion has been one from the time of Adam to Muhammad (peace be upon him), and that is Islam. Differences occur in the divine laws, and Muhammad (peace be upon him) is the Seal of the Prophets. His book abrogates the rulings that preceded it, while confirming the correct beliefs and narratives contained within them. - Answering his questions calmly, and if necessary, providing him with some books and video tapes by Sheikh Ahmed Deedat.