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 is His slave and Messenger.
With regard to Hajj, it is permissible for you or for anyone else to perform Hajj on behalf of the person whom you mentioned in the question on the condition that the person who performs Hajj on him (the dead) has already performed Hajj for himself; for more benefit, please refer to Fatwa 84509.
As for the wording of supplicating for him, then there are many supplications that were reported in the Book of Allaah and the Sunnah of the Prophet for seeking forgiveness for the dead or for the people who are still alive. Therefore, you may supplicate for him with the supplication which the Prophet supplicated with for Abu Salamah when the latter died. Umm Salamah said: "The Prophet entered upon Abu Salamah (when he died). His [Abu Salamah’s] eyes were fixedly open. So, he [the Prophet] closed them, and then said: “When the soul is taken away the sight follows it.” Some of the people of his family wept and wailed. So he, sallallaahu ‘alayhi wa sallam, said: “Do not supplicate for yourselves anything but good, for the angels say "Aameen (Amen)" to what you say. He then said: “O Allaah, forgive Abu Salama, raise his degree among those who are rightly guided, grant him a successor in his descendants who remain. Forgive us and him, O Lord of the Universe, and make his grave spacious, and enlighten it for him.” [Muslim]
As regards his statement that he wants to give his wife all his properties, then if there is evidence for this – if the heirs or two trustworthy witnesses assert this – then his words are considered as a will, in which case, you are entitled to take only a third from the inheritance [as you are only his fiancée and not his wife], and what exceeds a third cannot be valid unless with the consent of his adult and rational heirs. If you take a third, you may donate it on his behalf because it is permissible to give charity on behalf of the dead.
Allaah Knows best.