What about a story reader? About $20. He would benefit from the reading, it doesn't require a stylus to advance the page. When you hear a chime, that is when you turn the page. The regular story reader has many, many books, including Thomas. Obviously the story reader plus (with video) would not be beneficial to him, but I just wanted to mention because you can not interchange the books with the devices.
A little unconventional?
What about a jump rope?
School supplies, or backpack? Not exactly a fun gift. But you could personalize them with his name, in print, and then hand embroider or use fabric paint to add braille on top of on each letter.
Possibly one of those bouncy balls with a handle on it, if he has a good space in the yard or somewhere to use it. Target had some in junior size and regular in the summer stuff, and you can also get them online. I think one step ahead, but I'm not positive.
Also, one step ahead has inflatable indoor bouncy toy. Like the outdoor rental ones you see at parties, but on a much smaller scale. Good for inside or out. Around $100 though.
Best of luck, you're a thoughtful aunt. =)
Melanie