My boyfriend might be the father

I've been with my boyfriend on and off for almost three years now. After 6 months of dating we broke up for about 2 months during that time we were both seeing other people but then after that we got back together and I got pregnant at 4 1/2 months I had a miscarriage. A year later he finds out one of the girls he messed around with when we were broken up has a child and he might be the father. This killed me inside. I cried for days. Now he's taking care of the baby, and everytime I see her I'm just reminded of the baby I lost. He hasn't done a DNA test and he refuses to do one because his parents are telling him it's his child but I just want him to be sure because this girl was messing with someone else and she doesn't know who's the father.