SO doesnt want to tell anyone ...

I have a 10.5 month old DD and just found out I am pregnant again. I am really excited and can't wait to tell our parents/siblings. Me and my family are really close and I know they will be very happy... My SOs is a different story. We aren't married, so when we first told them about our first pregnancy they were shocked and didn't handle the news well. They did come around really quickly and were very supportive and completely adore their grand daughter. But now that we are pregnant again, my SO is feeling very anxious about telling his family. He thinks they will think it's too soon for another. What can I do to get ease his anxiety?

We have a birthday dinner coming up with both our families together ... I really wanted to show up with DD wearing a "Big Sister" shirt but SO keep avoiding the idea.