Guitars seriously vary in price, from budget makes available in department stores to custom models from famous brands. If your man is starting out, there are beginner packages available that include practice amplifiers and leads along with the guitar. If he is celebrating a milestone birthday or you have a large budget, then go for the big guns. Fenders and Gibsons are the most well-known high-end brands, and their American-made models are regarded as some of the best in the world. Try a Fender Stratocaster or a Gibson SG.
If he is serious about his passion and already plays the guitar of his dreams, then go for accessories. You could buy him a leather guitar strap with his name embossed on it, or a set of guitar picks printed with a photograph he loves. If he is a real enthusiast then you could present him with a set of custom-wound pickups. These will dramatically improve the sound of his instrument and make his "axe" truly stand out.
Even if he has never picked up a guitar before and only hinted at learning to play, surprise him with a set of lessons to go with his beginner's package. If he is self-conscious then arrange for one-on-one sessions with a tutor. If he is keen to jam with other beginners then group lessons are available. To inspire him, buy tickets to see his favorite band when they next come to town and present him with a song book, so he can play along with the album.
If your man is a confident guitar player and already in a band, then you can book time in a local studio to record his first CD. If he is technically-minded, then arrange tickets to a guitar convention or a workshop. The big brands like Fender, Gibson and Ibanez sell merchandise such as T-shirts and baseball caps. For the guitarist who has everything, then present him with super-high-quality power leads from a specialty store.