Some Mormons are visiting him and trying to make him doubt his religion

My dear brother, you have
to know with absolute certainty that the religion of Islam is the true
religion which is pleasing to Allaah, and that Allaah will accept no
other religion from anybody apart from Islam. Islam is based on the
worship of Allaah Alone with no partner or associate, and on following
the Messenger Muhammad (peace and
blessings of Allaah be upon him). This is the implication of bearing
witness that there is no god except Allaah and that Muhammad is the
Messenger of Allaah. 

If these beliefs are firmly entrenched in
the heart of the Muslim, he will not care about any opposition or mockery
on the part of others. Rather he will pray to Allaah and act in accordance
with the religion of Allaah, and he will say, “I am one of the 
Muslims.” Allaah says (interpretation of the meanings):

“And
who is better in speech than he who invites (men) to Allaah’s (Islamic
Monotheism), and does righteous deeds, and says: ‘I am one of the Muslims.’”

[Fussilat 41:32]

“Therefore proclaim
openly (Allaah’s Message
Islamic Monotheism) that which you are commanded, and turn away from
Al‑Mushrikeen (polytheists, idolaters, and disbelievers).

Truly, We will suffice
you against the scoffers,

Who set up along
with Allaah another ilaah (god); but they will come to know”

[al-Hijr
15:94-96] 

The Christians are among these mushrikeen
because they worship the Messiah and his mother, and they worship the
cross. So do not pay any attention to them or their mockery; turn away
from them and do not enter into debate with them, lest they make you
confused with their specious arguments. If you can change schools and
move to a school where you will not be exposed to this trouble, then
do so.

 The Mormon sect appears to be Christian
because they mention their book, which is the Gospel (New Testament),
and they say that if Islam were like this, i.e., opposing their religion,
it would have refuted their book. This is ignorance and confusion on
their part, for Islam does not refute the Injeel which Allaah revealed
to ‘Eesa; in fact the Qur’aan confirms it and the Books that came before
it. But Islam refutes the religion of the Christians who have taken
the Messiah and his mother as gods instead of Allaah, and Islam refutes
the distorted Gospels which are in the Christians’ hands. Allaah says,
after mentioning the Tawraat  (Torah) and Injeel (Gospel):

“And
We have sent down to you (O Muhammad) the Book (this Qur’aan) in truth,
confirming the Scripture that came before it”

[al-Maa’idah 5:48]

i.e., the Qur’aan confirms the Books which came before
it, such as the Tawraat and Injeel.

The Messiah has disowned these mushrik
Christians and their way, and he will disown them on the Day of Resurrection,
as Allaah says (interpretation of the meaning):

“Surely,
they have disbelieved who say: ‘Allaah is the Messiah [‘Eesa (Jesus)],
son of Maryam (Mary).’ But the Messiah [‘Eesa (Jesus)] said: ‘O Children
of Israel! Worship Allaah, my Lord and your Lord.’”

[al-Maa’idah 5:72]

“And
(remember) when Allaah will say (on the Day of Resurrection): ‘O ‘Eesa
(Jesus), son of Maryam (Mary)! Did you say unto men: ‘Worship me and
my mother as two gods besides Allaah?’  He will say: ‘Glory be
to You! It was not for me to say what I had no right (to say). Had I
said such a thing, You would surely have known it. You know what is
in my inner‑self though I do not know what is in Yours; truly,
You, only You, are the All‑Knower of all that is hidden (and unseen).

Never did I say to them aught except
what You (Allaah) did command me to say: “Worship Allaah, my Lord and
your Lord.” And I was a witness over them while I dwelt amongst them,
but when You took me up, You were the Watcher over them; and You are
a Witness to all things.’”

[al-Maa’idah 116-117]

We ask Allaah to make you
steadfast in Islam and to suffice you against the evil people. May Allaah
bless our Prophet Muhammad and his family and companions, and grant
them peace.