When to tell an older child I’m pregnant?

Those of you with older kids, how long did you wait before telling them you were pregnant?

After 2 years of trying, I am 6 weeks pregnant and we are really struggling to keep it from My 8-year-old stepson. He and I spend a lot of time together, I have a great relationship with both him and his bio mum. He is ridiculously intuitive and can smell changes from a mile away. He lives with us 50% of the time and has started to question things like my change in routine and why I won’t “roughhouse” anymore (we used to have teddy wars most days and it could get a bit intense - so stopped when I fell pregnant). He is also aware we were trying for a baby and we explained infertility and endometriosis to him because he wanted to properly understand why we couldn’t give him a sibling yet.

Waiting another 6 weeks is going to be near impossible but telling him now feels too soon. Any recommendations?