He was Muslim, then he went back to his country and apostatised, then he came back to a Muslim country; is he a mu‘aahad (non-Muslim living under Muslim protection)? How should we deal with him?

Firstly: 

If a person was a Muslim, then he apostatised from Islam and
became an atheist, he is an apostate, and there is no confusion about that.
The one who apostatises from Islam to another religion is not regarded as a
follower of that religion and does not come under the same rulings as
followers of that religion. Rather he is an apostate, like the atheist
mentioned above, and is to be treated as such. He is to be asked to repent
and come back to Islam; if he repents and comes back (all well and good),
otherwise he is to be executed because of his disbelief (kufr). 

Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allah have
mercy on him) said: 

If anyone apostatises and becomes a Jew or a Christian, we do
not accept that, because the Prophet (blessings and peace of Allah be upon
him) said: “Whoever changes his religion, execute him.” Narrated by
al-Bukhaari (2854). In other words: whoever changes from being a Muslim, we
should execute him. 

Ash-Sharh al-Mumti‘ ‘ala Zaad al-Mustaqni‘,
11/306 

Based on that, if a person was Muslim then he became a
Christian or a Jew, meat slaughtered by him is not permissible because he is
not a kitaabi (one of the People of the Book); rather he is an apostate
whose meat cannot be eaten. If such a person is a woman, it is not
permissible to marry her, for the same reason. These people cannot be dhimmi
or mu‘aahad or musta’man (one who is granted protection and security by the
Muslims), because these statuses can only be granted to one who was
originally a kaafir, not to an apostate. The one who apostatised from Islam
has no choice but to return to his religion or to be executed as a kaafir,
if he persists in his apostasy. 

Shaykh Saalih al-Fawzaan (may Allah preserve him) said: 

In shar‘i terminology, the apostate (murtadd) is the
one who disbelieved willingly after having been a Muslim, by words, beliefs,
doubts or actions. 

There are rulings concerning the apostate in this world and
in the hereafter. 

With regard to this world, it was explained by the Messenger
(blessings and peace of Allah be upon him) who said: “Whoever changes his
religion, execute him.” The scholars are unanimously agreed on that and on
what results from it of his wife being separated from him and his being
prevented from disposing of his wealth before he is executed. 

With regard to the hereafter, that is stated by Allah, may He
be exalted, in the verse (interpretation of the meaning): “And whosoever
of you turns back from his religion and dies as a disbeliever, then his
deeds will be lost in this life and in the Hereafter, and they will be the
dwellers of the Fire. They will abide therein forever” [al-Baqarah 2:217].
Apostasy occurs when one commits any of the actions that nullify Islam,
whether that is done in earnest or in jest or in mockery. Allah, may He be
exalted, says (interpretation of the meaning): “If you ask them (about
this), they declare: “We were only talking idly and joking. Say: Was it at
Allah ( ), and His Ayat (proofs, evidences, verses, lessons, signs,
revelations, etc.) and His Messenger (SAW) that you were mocking? Make no
excuse; you have disbelieved after you had believed” [at-Tawbah 9:65]. 

Al-Mulakhkhas al-Fiqhi, 2/565,
566 

See also the answer to question no.
20060 for further information. 

We should point out that the hadd punishment for apostasy –
like all other hadd punishments – cannot be carried out except by the caliph
or his deputy, according to scholarly consensus. It is not permissible for
individuals to carry out the hadd punishments themselves. 

We have discussed this in the answers to questions no.
107105,
12461 and
8980. 

Secondly: 

With regard to the way he should be dealt with, that is the
same as any other disbeliever: he should be called to Islam with kindness,
reminding him of his past when he was a Muslim, and that he must inevitably
feel the difference between his life in Islam and his life outside of it.
Help may be sought in doing that from those who know his language and come
from his country, so that it will be easier for him to understand. Giving
gifts also has a good impact on people’s hearts, so give him things that he
needs, by means of which you will be able to reach his heart. If he has some
difficulties in some worldly matters you can help him with them and make
things easier for him. All of that will make him think again about what he
has done, and perhaps he will come back to Islam before he dies, and that
will be recorded among your good deeds. 

All of that has to do with interacting with him. With regard
to the ruling, we have stated above that he is not like one who was
originally a kaafir. It is not permissible for a Muslim to inherit from an
apostate or for him to inherit from a Muslim, and the apostate woman cannot
marry a Muslim. If a Muslim woman is married to the apostate, her marriage
contract is rendered invalid. And there are other rulings. But we should
differentiate between the rulings and how we treat the individual. 

And Allah knows best.