I always train my own dogs first with hand signals and second with verbal commands, if at all. That way I can be sure that my dog will actually look at me for input, instead of looking at some distraction. (One of my first pooches was deaf, so that may be what started it. But hey, it works!)
The reason dogs respond so much better to hand signals is pretty simple. Let's put it this way: how many times have you seen a group of dogs standing around in a circle saying, "Woof woof woof?" and agreeing, "Woof!". The majority of canine communication is based entirely around visual signals and body language. Barks, growls, howls, and whines are also communications, but they are blunt and ineloquent when compared to the vast range of exchanged that can be passed along through body language. People are an extremely verbal species, so it's hard for most of us to understand the dogs' world of mostly silent language. But it makes sense that a dog will differentiate one hand signal from another long before he can tell the difference between words.
So to answer your question, Renee nailed it. If his hearing's okay, then you just need to couple the verbal cue, which he doesn't know, with the hand signal, which he does know. Then slowly take away the hand signal, and reward him when he obeys the verbal cue by itself.