You have some really good suggestions. I have a couple more to add.
My DS got some movie theater gift certificates last Christmas and that was a big hit. That might be a fun one to add, especially if you think the family might have a movie that you'd all like to see over the holidays.
I get my kids to give me their game lists early. Then I do searches on Ebay and buy them used. The kids don't care if they are new or not, as long as they work. They know that I can buy them more if I buy them gently used, and I have had pretty good luck. I've never tried EB. I'll have to check them out.
Go out and get some things at
Walmart, like a yo-yo, playing cards, a frisby, etc that don't cost much. You wouldn't have to spend much to help fill the space. Does he like movies or music? Sometimes before the holidays they have great sales on DVDs and CD's. These can also be purchased used at great prices.
My son also loved the t-shirts from all of the islands on our cruise last year. You might want to consider hiding some if you think he'd be interested. Del-Sol shirts and hats that change color in the sun are really cool. They have shops at St. Maarten and St. Thomas. Not really cheap, but not terribly expensive either and my kids thought they were great!