Our DS1 was 8 when we adopted DS2 last year. Ours situation was a quick one, found out about baby on Tuesday, brought him home on Wednesday. We didn't tell Ds about the baby till he got home that Wednesday after school and we went to pick baby up, by that point TPR was signed. Our situation was pretty much a "sure thing" but I couldn't chance telling DS1 any sooner just in case. Plus, we live in a small town and DS has a big mouth (in the sweetest way of course, LOL), he would not have been able to keep that secret at school
.
We had to do lots of whirwind shopping so I got gifts for older DS, and gave him a card that was "from his baby brother". It was so much fun watching him read it and trying to figure out what was going on. When he finally figured it out, he immediately said he was going to teach his new brother to "blow spit bubbles". That's boys for ya!
Hope things go smoothly, would love to hear what you decide to do.