We always need to take lessons from realities of life and study the majority of similar cases before emotionally going ahead with marriage proposals based on promises. Lot of marriages have taken place based on a promise to become a Muslim but ended up with different situations few years after the marriage. Emotions before marriage usually don't last long after marriage. Usually married couples face some issues and misunderstandings between them which can decrease their emotions towards one other. Studying the results of thousands  of similar cases should make us more careful before deciding as such decision is for life. As many similar promises were not fulfilled properly by many, we need to be sure that person has really accepted Islam for the sake of Allah and decided to follow the True religion. His real practice is must and not his promise. After he really practiced Islam like a good Muslim, then you can assess whether he is willing to remain a Muslim for life or just for the sake of marriage.

In fact just promising to be a Muslim will not make permissible for him to marry a Muslim girl, unless he is already a Muslim.

You need to be sure about your future and the future of your children.

Wassalam.