My boyfriend still sees his ex girlfriends kid(the kid is not his) is this weird? ?

Ok my boyfriend before he was my boyfriend dated this girl for 3 years she had a baby like 6 months before they got together so the baby is not his but the dad is out of the picture, so they broke up about a year and a half ago and she is engaged to someone else and my boyfriend and I have been together for 10 months, is it weird that my boyfriend still sees his ex girlfriends kid once a week? I don't have any experience with something like this but I think its weird that she still let's them see eachother for that long after they stopped dating and she is engaged now, I understand that my boyfriend doesn't want to walk out on the kid like his dad did but how long is too long? And how am I supposed to handle this? As far as I know he(my boyfriend) only talks to the ex girlfriend when it is about the kid but I don't really know, and it makes me nervous.