Here is what I ended up writing:
"Dear ACME Family,
It is almost time for dear John's promotion from being our boss to brother for all of us.
The barbecue on the 19th will be the means to wish him all the best in his new chosen path of his life journey.
We thank him for building this successful company that many envy, for choosing us as his team mates and we all wish, his decision will carry him on to health, happiness and joy filled new adventures.
Taking the opportunity, from the bottom of my heart, I wish all of us that this important change in our lives will be the best for all."
Writing these simple sentences took me a good half hour.
Everybody chipped in for a gift and I added the extra required to buy him a Boss watch from all of us and we are making a huge card with photos and our signatures to be given to him on the 19th.