Love and exes

Okay, so I've never been one to worry about exes but I've been dating my partner for 7 months and I found out one of his exes has a child & did have when they dated (not to my partners child) 
I don't know how to feel about this, in a way I'm like oh well the past is the past but in another sense it's like I wish you told me. 
Do I have a right to feel like this?