Even though you are from Detroit they would probably love NYPD type of stuff since nueva york is the capital of the U.S. to most Dominicans. You could also bring them perhaps so portable CD players so the boys could listen to their music, like jazzcom said anything that has to do with Domincan baseball players is cool, even if it's a cubs baseball hat. If they are into baseball you could buy them a glove and bat and a couple of balls.
Coffe table book is a good idea, you could also put together a photo album for his parents of you two and the types of places you have been and pictures of him in different places that way they can show him off to others when people ask about how their son is.
Also maybe a nice table cloth for the dining room table, or a fruit bowl or vase of some sort where you could place either fresh or silk flowers, women love that type of stuff, and you could bring the father a nice colgne or bottle of whiskey (that you can pickup at duty free)
