Rather sadly, a former brother converted to Christianity and this information has been verified. What options are available to us now in dealing with him? Should we boycott him or continue to associate with him? What was the way in which Abu Bakr dealt with the murtaddoon?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

We advise you to continue to advise your friend, to explain dangers of his deed and apostasy and the essentials of Islam and its virtues. You have to tell him that Allah will not accept any religion except Islam. Allah says: {And whoever seeks a religion other than Islâm, it will never be accepted of him, and in the Hereafter he will be one of the losers.}[3:85]
Then, you have to demonstrate to him that Christianity has been distorted, changed, and finally abrogated by Islam.
Allah says: {And We have sent down to you (O Muhammad SAW) the Book (this Qur'ân) in truth, confirming the Scripture that came before it and Mohayminan (trustworthy in highness and a witness) over it (old Scriptures)...} [5:48]
If he accepts that he will benefit; otherwise you have to desert him if this is beneficial. You have to avoid thinking of killing him since only the Muslim ruler has such a right. And if there is a Muslim ruler, we advise you to inform him of person's state.
The Prophet (Sallallahu Alaihi wa Sallam) said: "whoever changes his religion (i.e. apostatizes), kill him." [ Bukhari ]
Thus, Abu Bakr (RAA) waged a war against apostates and took their money and captured their woman and children. But, after that incident scholars agreed that it is not permitted to take women and children of apostates as captives.
Allah knows best.