Fiancée hid child from his previous relationship. What should I do

Why don't you give us more details into your situation.

Talk with him? Why did he hide it? Did he think it would be a deal breaker with you?

Have you asked him why he did it?

You have to express to him what you're feeling. You CAN'T accuse him or berate him, you need to talk to him about how this makes you feel and how it will affect your relationship going forward. You're going to be a step-mother, so you also need to prepare yourself for that, if you continue on to the wedding.

How long have you both been dating... a week? Ten years? Details matter.

How long have you two known each other?

He's your fiancé, so I'm assuming you've been together for a long time He never once mentioned having a child in all that time, before he asked you to marry him?

I would ask him why he hid it. Obviously he doesn't see the child, or if he does, he never let you meet the child while he had visitation. Does he support the child?

We really need more information. Based on what you posted, we can guess about the relationship, but without more info, all we're doing is guessing based on our own perceptions, and giving advice based on those perceptions. In other words, you won't be getting valid advice based on your actual circumstances, without providing more info for us to go on.

There may be several things,
One he may never see the child, has not contact, in which case, he did not see the child as an issue.

He doubted paternity and it has never been proved?

He felt that you may not accept him, knowing it, and just did not tell you.
I would have to look toward the first choice, if, you are engaged, and you did not know he was spending weekends with child, summer with child, then it appears he has no real contact and bearly feels it is important.

The issue is, why is it an issue? Why is it bothering you?

I am curious to know what his excuse was for not telling you from the beginning, since that had to be the first question you asked when he did finally tell you. That leads to my second question of why you didn't/don't believe him?

Kind of unfair to ask strangers to form an opinion with so few facts, least of which is how long this relationship has gone on that leads to an engagement?

Or did you find out about his child through the grapevine, and haven't even discussed it? Not getting his side of it would be a huge problem in itself in my book.