make sure you understand first if he wants a great big chunky watch like a Rolex (which will also wear his shirt cuffs to tatters) or a slim elgant watch like a Cartier.
Or a high-tech watch with lots of buttons and dials like a Seiko.
Or a Submariners or Divers watch
Or a simple but very tough watch like the British forces use (made by CWC and not by other makers)which does also include Divers watches in their range)
Find out if he wants a showy dial or a black and white one (which is far easier to read), and if he prefers a metal bracelet or a leather strap. See if he thinks he wants a Date or Day indicator (these are, in practice, very little use), and if he thinks he needs it to be really waterproof.
You do not actually have to go for the brand names I mention, but make sure you know what style he wants, and then see what suits your budget in that style. If the people he mixes with wear flashy watches, ask him what sort, and which he prefers (among a certain band of saleman, getting your first Rolex is known as geting a "badge").
Having had a few, I would actually recommend getting a watch with a Quartz movement rather than a hand-wind or automatic watch. I've got an automatic, and it is surprisingly tiresome these days having to wear or wind it every day, and finding it stopped or slow on a Monday morning if you didn't use it over the weekend.