If I cannot establish proof against him because he followed his whims and desires, or because I am not qualified to do that, should I treat him as an apostate or not?
The apostate is the one who has gone out of Islam and become a kaafir (disbeliever) by his words or actions, or by his giving up (some obligatory duty) or his beliefs.
Not everyone who falls into kufr is regarded as an apostate kaafir. There are some reasons for which a Muslim may be excused and thus not deemed to be a kaafir, such as ignorance, misinterpretation, compulsion and error.
See the answer to question no. 4231
And there are some types of apostasy for which there is no excuse, such as reviling Allah, may He be exalted, or reviling His Messenger (blessings and peace of Allah be upon him), or clearly expressing heresy, because the common folk and the elite are unanimously agreed that this is kufr and apostasy from Islam. The one who does any of these things and does not repent is an apostate.
If it is proven for certain that a person is an apostate, and he is a relative, then he is to be treated in the following manner:
1. It is obligatory to disavow whatever he believes in of kufr and apostasy.
2. It is haraam to regard him as a friend and love him.
3. It is obligatory to advise him and call him to come back to Allah, may He be exalted.
4. It is permissible to visit him, talk to him and sit with him with the aim of calling him to Islam and seeking to guide him, for the one who is qualified to do that.
5. It is permissible to uphold ties with him by giving gifts and the like, to encourage him to repent and follow true guidance.
6. He should be shunned and cut off if he persists in his misguidance, especially if there is an interest to be served by shunning him, such as exposing his evil or to discourage the other relatives from following him.
To sum up, you know that this apostate is despised by Allah and Islam does not allow him to remain in his apostasy. By apostatising he has lost all the sanctities and rights that he had over the Muslims. Calling people to Allah, may He be exalted, and being eager to guide people are among the greatest of good deeds and the most beautiful of characteristics. So interactions with him must be based on these two principles, hating the kaafir because of his religion and calling him and trying to save him from his kufr.
And Allah knows best.