A nice warm cardigan in moss green or maroon is always a nice gift.
Does he need any antimacassars? They're so attractive in the parlor.
Enough of that.
If Sam were graduating and depending on my budget, I'd get him an iPhone with a year's service or iPod with a $100 iTunes gift certificate to buy apps, movies, and stuff like that. I find my iPod pretty indispensable. He can Twitter with it, keep a calendar, has Yelp!, calculator, alarm clock, GPS, and about a zillion useful apps like a gas price finder, dream interpreter, weather channel, news, take pictures and video, and everything else you can imagine. There's even an app to use it as a flashlight! Besides all the use he'll get out of it, it's a cool thing that an 18 year old would appreciate. With a car adapter he can use it to listen to music in his new car too!
I think my iPod was one of the best things I've ever purchased given how much use it receives and the amount of information it can give me. The only problem with the iPod is that it's dependent on wifi where the iPhone is not. That makes the iPhone significantly more useful than my iPod and yet I'm not displeased with my iPod at all.
I'd also give him a big hug and tell him how proud you are of his achievement and how much you love him. It's a cheap gift but very meaningful. To go with it, I'd give him a little something personal from you. Maybe cufflinks or something. Though those kinds of gifts are rarely used, for the rest of your life you never use them without thinking of who gave them to you. If you need a pocket watch to give him, let me know. I have one you're welcome to have.