I would host an intimate dinner party for a few people in his honor. Wine, a nice dessert, a good selection of music, the whole shebang.
[I never would have thought to buy a gift for a family member that was retiring. I've attended work luncheons in honor of someone retiring, but that was because we were coworkers. Is this common elsewhere? Of course, I've also never understood celebrating other people's wedding anniverseries, except for some big milestone like 50 years. My parents celebrate their wedding anniversary between the two of them. I find it odd to get a happy anniversary card from someone I'm not married to, but maybe that's just me.]