He is not sure whether he uttered the shahaadatayn, two years after he became Muslim

Firstly: 

We praise
Allaah for having guided our brother to Islam, and we ask Him, may He be
glorified and exalted, to make him steadfast, strengthen his faith and
increase his worship.  

Secondly: 

If this
brother is doubting that he uttered the shahaadatayn after two years of
fasting, praying and striving to follow the religion as much as he can, this
is undoubtedly waswaas (whispers) from the shaytaan who wants to upset him
and make him feel anxious and worried. What is definite is that this brother
has repeated the shahaadatayn during this lengthy period time after time.
Has he not been repeating the adhaan after the muezzin all this time? Has he
not said sometimes Laa ilaaha ill-Allaah Muhammad Rasool Allaah? Does
he not pray? For in the tashahhud during the prayer he says: I bear witness
that there is no god but Allaah and I bear witness that Muhammad is the
Messenger of Allaah. 

Moreover the
fact that he prays is regarded as entering into Islam. Hence the scholars
said of the kaafir: If he prays then he is a Muslim according to the rules,
i.e., he is judged to be a Muslim even if he does not utter the
shahaadatayn. 

See:
al-Sharh al-Mumti’ (2/12) and al-Mawsoo’ah al-Fiqhiyyah (4/272). 

This is also
indicated by the report narrated by al-Bukhaari (391) from Anas ibn Maalik
(may Allaah be pleased with him) who said: The Messenger of Allaah
(peace and blessings of Allaah be upon him) said: “Whoever prays as we pray
and faces the same qiblah (direction of prayer) that we face and eats meat
slaughtered by us, he is the Muslim.” 

We should
beware of waswaas which befalls some people and makes them confused as to
whether they said or did something or not, because waswaas is a sickness,
and the remedy for it is dhikr (remembering Allaah) and not paying any
attention to it. 

See also the
answer to question no. 62839. 

And Allaah
knows best.