I really don't understand people comparing him telling in boss - within minutes of an uncertain test, knowing you wouldn't want him to, and you telling your sister - a week later, with his knowledge and agreement.
I do understand the telling in terms of each of you needing support - and perhaps a boss you don't know is a good person for him to confide in, if you don't have a relationship with this person it doesn't compromise your needs. But, he should have waited at least until you'd seen each other following the test, were certain it was real and had had a conversation about who you were telling when. You'd shared the test result in good faith believing you could trust him. From then on it was your shared result, not his to do with as he pleased.
I very much understand your desire to be certain before telling anyone. I had a miscarriage last year (10 weeks, pretty much took it in my stride as just something that happens). DP and I are the only non-medical people to know about that (no one's banned from mentioning it but we're both fairly self-contained people and haven't felt a need). Two months later I conceived again, now 31 weeks. We didn't tell people, parents included, until 20 weeks because I wanted to be as certain as possible, in possession of as much information as possible and I didn't want to raise false hopes. Yes, I am generally quite private, discrete and am not very close to my closest family, so I recognise that I'm not a 'normal' example. But, despite feeling 'ok' about the miscarriage and knowing it had no significance for this pg, I did feel very much more cautious as a result.
Best wishes for yours, try to stay calm and happy for you own sake - perhaps even if that means letting go of some frustration. Perhaps just ask your DH to tell you about his feelings and listen without commenting, then mull over? Sounds like he wants to tell you.