Can anyone offer me some good advice??

Ive been with my partner on and off for 4 years now and in that time, we"ve dealt with depression (both of us), unemployment, cancer, and then myself having heart surgery. The stress of it all eventually caused us to split up 18 months ago. My partner ended up seeing someone else who went on to have his child even though he was told he would be infertile after testicular cancer and we had been trying too, but no success. Three months ago, he got back in touch and asked if we could try again and although we have been friends up until now, the feelings have come back and i know i still love him and vice versa. However, he has also now decided that he wants to know for definite if this child is really his and is meeting up with the childs mother tomorrow to discuss the paternity. Im angry as she is back in our lives again but i understand him wanting to know if the child is his. Im feeling really hurt and angry right now as i feel threatened. Can anyone advise me on what to do here please