An Illuminations cruise is great! We did it for my FIL's birthday (went without the birthday package) and he loved it!
Maybe do what we did and start the evening off with a nice dinner at Yachtsman Steakhouse and then a short walk to where you pick up the boat for Illumiations. He had his cake at dinner.
Or if you want to get him something
DVC related the shops at all DVC resorts (VWL, BWV, BCV, OKW) all have some nice (but limited) DVC merchandise. Logo mugs, jackets, polo shirts, t-shirts, etc. That way he can tell the world he's a DVC member!