Smells can bring extremely strong emotions and draw connections that can't be overcome. The last thing you want is to smell like someone's dad, which is exactly what will happen if you go with Aqua di Gio (or like a Middle Eastern huckster, which is what you get if you pick Dior Sauvage).

The best thing you can do is to go to a store (Nordstrom, Ulta, etc.) that has samples, and smell them. Pick something that is newer and that "smells like" the person you're buying for.

I'd recommend starting with newer fragrances, like:

• Jean Paul Gaultier - le Male Elixer or Ultra Male
• Versace - Eros
• Parfums de Marly - almost anything they make
• Creed - almost anything they make, especially Aventus (but smell them first to make sure you're not picking an "old man" scent)
• Mont Blanc - Legend, Legend Spirit, or Explorer
• Tom Ford - Tobacco Vanille

I'd stay away from Chanel. Bleu de Chanel is a little bit older (popular with people who graduated college 10ish years ago) and remains so popular that it's guaranteed to smell like 4-5 other people (and in terms of longevity, it's pretty weak—it really doesn't last very long, which is absurd given how expensive it is).