All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
First of all, we would like to draw your attention that the acquaintances that happen between young men and young women and what is known as love relationships between them is a door to evil and corruption; for more benefit in this regard, please refer to Fatwa 363454.
Our advice to you is that if this man is righteous, then accept him as your husband. If you do not accept that he marries another wife, it is permissible for you to stipulate in the marriage contract that he should not marry a second wife. In which case, if he marries again, it is your right to invalidate the marriage.
For more benefit in this regard, please refer to Fatwa 84251.
If he does not consent to this, then consult some wise people from among your family or other righteous people, and balance between the benefits and harms in marrying him in that case and not marrying him, and choose the best for you, after performing the Istikhaarah prayer.
Allah knows best.