Rosie please don't let people here guilt trip you into telling your DP.
You do whatever is right for you.
Personally I wouldn't in your circumstances. Yes it'll be shitty for your DP if he finds out after the birth that it's not his baby. But it'll be shittier for your baby if s/he loses the chance to grow up with their father around because you split up before the baby came, and it turns out it was his all along.
I'm not saying the above would happen to you! But that it's what would worry me, and I think it's a perfectly legitimate concern.
There are three people's feelings to take into account here, and the baby is totally dependent on you making decisions for them, and also has the most to lose in this situation. I think people tend to focus on the partner's needs above that of the baby, and both are important.
Also what's the point in putting your partner through all the worry you're going through? If it was me I'd be tempted to bear that on my own, then share it with him only if it turned out the baby wasn't his. Why create all that drama and upset unnecessarily?
That's just my opinion and I know others here disagree with it.
You need to decide what you think is right to do, but don't let anyone here push you into doing something (including me!) and especially not if it's because they're making you feel guilty.
You have to live your life, not them, and you know the people involved. You do what you think is right for you, your partner and your baby..