When to tell son about becoming big brother

We had our first ultrasound on Tuesday, baby is good due August 21! We’ve had chemical pregnancies (two in the last two years) and one eptopic pregnancy about eight years ago, since seeing the baby and knowing everything is okay we are finally about to share the news with family, but I just don’t know when the “appropriate time” to tell our six year old would be.... he has been talking forever about how much he wants to be a big brother, but so many people have told me to wait until I’m further along, which makes sense, but at the same time God Forbid if something were to happen, technically it could happen at anytime during this pregnancy. Anyone out there who’s got to promote kids to big siblings.... when did you tell them?