It all depends upon what he already has. A good truck specific GPS is handy, but not all drivers like them. If he enjoys listening to audio books, they are always a welcome gift.
A lot also depends on how much you are wanting to spend. A good laptop with a wireless broadband card comes in very handy.
A good DVD player is nice to have, as is a flatscreen tv.
I love having my iPad along with me as I can use it to read books, I read a lot, as well as for getting online.
A new law went into effect this month requiring all CDL drivers to only use a hands-free cell phone. Does he have a good phone, like an Android phone, with a truck specific bluetooth headset like a Blue Parrot, Blue Eagle or, my personal choice a Blue Tiger? Android or iPhone is nice because it can do voice dialing, thus saving him from a very high fine if he's caught dialing the phone by hand.
I am just tossing out suggestions as I know nothing about him personally. These are all things I have in my own truck. Also, a gift card for a hotel room here and there isn't a bad idea.