When should I tell my son?

Angie • Married & pregnant with baby #2 expecting our beautiful baby girl August 21!
I'm having trouble deciding when to tell my son! He is 4 years old and has been asking for a brother or sister for awhile now. I am dying to tell him but I'm only 6 weeks. I had my first appointment yesterday and it was confirmed. My next appointment is jan 16th and we should be able to hear the baby's heart beat. Should I wait until then? It's so hard not to tell him! LoL I'm curious how far along women have waited to tell their other children. Please share!