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.
If the situation is truly as you mentioned in the question, then your fiancé is committing a number of prohibitions. Therefore, he should be reminded of Allaah and advised to repent to Him, especially about the matter of prayer if he is negligent as he might repent. If he repents, then praise be to Allaah, but if he insists, then, in our view, it is more appropriate to cancel this engagement with him because we fear that he will influence you after marriage and he will spoil your religion instead of you correcting him.
It is for this reason that the Prophet advised us when marrying off women (under our guardianship) to choose a man who is religious and has a good moral conduct, as he said: “If a man whose religion and moral conduct are pleasing to you proposes to you, then let him marry; otherwise there will be great mischief and corruption on earth.” [At-Tirmithi]
It should be noted that an engagement is just a promise of marriage and it is permissible for either party to terminate it at any time.
If we presume that marriage takes place, then you should endeavour to correct your husband by supplicating to Allaah to enable him to repent. There is no doubt that patience and gradual process are required so that rectification takes place, and you should concentrate on increasing his ‘Eemaan (faith) because if ‘Eemaan becomes firm in the heart, it becomes easier for a person to abide by the religious obligations.
On the other hand, it is not permissible for you to help him or obey him regarding an act of disobedience, otherwise you would be sinful. The Prophet said: “Obedience is required only in what is permissible.” [Al-Bukhari and Muslim]
Finally, it should be noted that your fiancé is a non-Mahram to you until the marriage contract takes place, so you should not talk freely to him; rather, you should not speak to him except in case of necessity or dire need. Also, you should not speak to him in a soft voice and if you speak to him, this should not happen when you are in seclusion with him. For more benefit, please refer to Fatwa 81356.
Allaah Knows best.