pregancy & new boyfriends

Hey so I just found out I was pregnant a couple of weeks ago well shortly after I met a guy and everything is going fine but I've yet to tell him I'm pregant I went to his house the other day and pulled out my ultrasound picture as a joke to see his reaction and he said we might as well not be together if this is true i don't want to loose him and being that I'm still not sure what I'm gonna do as far as the pregancy should I tell him or not ? I don't want to make him upset or loose him