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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The matter is as you mentioned, that that man has no relationship with your son, as he is not his legitimate father. So it is not forbidden for you to prevent him from seeing him, as he has no right to do so. This may also be a means for temptation and corruption as you mentioned that he wants a relationship. Please refer to Fatwa 102596.
If this man is a Muslim and you are not yet married and he made repentance and became righteous, and he wants to marry, then in this case, it is acceptable to accept his proposal of marriage. Otherwise, you should look for another righteous Muslim man to marry you in order to keep yourself chaste and live under his protection and guardianship. For more benefit, please refer to Fatwa 82471.
Allaah Knows best.