You should do something with him, instead of getting him something. Like you said, he's rich, he could buy something if he wanted it. But if you do something together, you guys'll have fun and you'll have the memories and everything.
What does he like to do? Besides the obvious. Like, is there any event that he would like? I know you guys just had a huge vacation, but it doesn't even have to be that big.
Like, this sounds crazy, but for Eddie's birthday last month, we went to a monster truck rally that just happened to be going on that night. And neither of us really have any interest in trucks or anything, but it was ridiculous and we just had a lot of fun. It's something neither of us would've really done on our own, but we had a great time together.