When to tell my son?

TS

My son is 5 yrs old, almost 6 (birthday is 3/2). It took us over 2 years to conceive our 2nd (with an early miscarriage along the way). I obviously don’t want to tell him just yet, since I’m only 4-5 weeks preggers. But I do want to tell him when I tell our families (grandparents), hopefully after my first ultrasound at 7-8 weeks....if all goes well. I know he will be so excited to finally have a baby brother or sister. But then I worry about him blurting it to others.

Any advice?