I told him as long as he followed these points of advice when he did speak up, his strategy would continue to serve him well....
1. Use strong language (I believe this, I strongly feel this, etc.)
2. Use inclusive language (This board's strategy is to, Our place is to do, I know we want to, etc.)
3. Be brief and comprehensive (This demands a strong pre-filtering capability of getting right to the heart of the matter)
4. Talk in lists (This will help keep it brief and give others a tracking mechanism)
5. Recognize and point out situations where he is speaking straight from the gut (not much identified supporting data) or vice-versa (not a strong personal belief one way or the other, but a good amount of supporting data).
6. Make certain you do speak up when you feel strongly about something or if you are the resident expert. You can't move up the chain of command just by doing good work. You have to let the right people know about it, also.
After discussing these strategies and validating his current approach, he is now starting to see that his current passive approach to speaking up in high-level meetings actually isn't a weakness. And it won't turn into one if he follows the points above.