How can a 16-year-old friend invite his Christian friend to Islam and convince him of it?

A Muslim must be keen on righteous companionship that helps him draw closer to Allah, reminds him of Allah, and strengthens his resolve; for the Prophet, peace and blessings be upon him, said: "Do not accompany anyone but a believer, and let no one eat your food except a pious person," and he also said: "A man is upon the religion of his friend, so let one of you look at whom he befriends."

It is recommended for him not to associate with non-Muslims unless it is for the purpose of inviting them to Islam. Among the means of dawah (invitation) is to gift them a translation of the meanings of the Noble Quran and books that clarify the virtues of Islam, to guide them to visit dawah websites, and to treat them well; because good example is one of the best means of dawah.