How do I tell my Step-son that I’m pregnant?

I (30F) am pregnant (8 weeks). Me and my husband are planning to tell my 6 year old step-son that he’s about to get a little brother or sister. He’s an amazing kid and we both get along very well. I love him. Once in a while he asked about when will we have a baby, because he wants to be a big brother. But, after he asked that cute question, he stars to cry and says thing like “I want a little brother, but my father will stop paying attention to me and he won’t love me anymore”. That worries me.