A letter from each kid expressing their pride would be priceless.
Or video with a clip from each one. (proper preparation for each kid would be important).
I think something tangible, rather than a cake that's soon gone.
Even a gift not shas related.
breifcase, wine cooler, golf clubs. whatever would be something he'd enjoy or feel special having. Bonus points for something he really wants, but wouldn't splurge for.