I'm not sure what "cut me off" means - are they currently supporting you by providing a place to stay, money or child care? If so, they have the right to put strings on that support and if you choose to disregard their feelings on the matter be prepared for the consequences.

If they are not supporting you, then it's less crucial if they cut you off, but again they have clearly stated they will not have a relationship with you if you go back with him. Again, just a consequence.

Choose wisely. If this is a longstanding on-and-off type relationship where you have shared some negative experiences with this person with your family, they are probably tired of the drama and are going to end it if you go back. It's their right.

You CAN co-parent without being in a relationship with someone.