One of my relatives denies that I am the son of my father without a valid reason, can I boycott him? Can the case be taken to a Sharee'ah court, taking into consideration that he was extremely infuriated when he said that?
On the other hand, what is the ruling on a Muslim who went to church with a Christian woman in order for her to offer the prayer, knowing that there is no legal relationship between them, i.e. he is not her husband or brother; rather he is her classmate?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

If that man's words imply explicit denial of your attribution to your father's lineage, then this is a slander that entails being lashed with 80 lashes. It is permissible for you to refer the matter to the Islamic court, if there is any, to carry out the corporal punishment upon him. Ibn Qudaamah said: “If a man denies the attribution of someone to his father's lineage, then the prescribed corporal punishment is to be carried out upon him. This was stated by Imam Ahmad."

Ibn Qudaamah substantiated this ruling with the words of Ibn Mas‘ood as he said: "There is no lashing except in two cases: A man who slanders a chaste woman or a man who denies someone's attribution to his father's lineage.”

This is not the opinion of Imaam Ahmad only; rather, it is also the opinion of all scholars of earlier and later generations. This was stated by Ibn ‘Abd Al-Barr in Al-Istithkar, where he said:

"There is no disagreement among the scholars of earlier and later generations that the one who denies that someone belongs to his father's lineage while his mother is a free Muslim chaste woman has to be lashed 80 times. However, they held different opinions about the ruling if the mother is a slave or a thimmi (a person from the People of the Book who lives in a Muslim state)."

As for insulting him in the same way as he did, it is impermissible. That is because this entails slandering his mother, who did no wrong against you. Also, it is not permissible for you to carry out the prescribed corporal punishment upon him by yourself because doing this is the responsibility of the Muslim ruler.

 It is not permissible to go to church along with a Christian woman because her prayer is error and falsehood, and it is not permissible to help or encourage someone in doing so. Rather, it is feared that such an action is indicative of the person's approval of the disbelief of the disbelievers, while scholars have regarded that as a form of disbelief.   

Allaah Knows best.