Seiko is always a good place to start. A Seiko diver.

There’s a few things to be aware of: Size. Most dive watches have a diameter of between 38mm and 44mm. The diameter is the standardised way at which a watches size is described.

If he’s a small / slight guy try something around the 38mm to 40mm size.

If he’s of average size 40mm to 42mm would be better.

If he’s a big fella, 42mm to 44mm.

Seiko generally wear small because they have short lugs (the protrusions at each end of a watch where the bracelet / strap attaches) so you’ll probably be ok no matter what you choose - but it’s best to err towards smaller rather than larger, but irrespective of the diameter of the watch there is another measurement that is more important to how a watch wears.

The ‘Lug to Lug’ length is the most important metric. That’s the distance of the watch from end to end; from lug tip to lug tip. Don’t buy anything under 44mm if he’s a big guy, or anything over 47mm if he’s a small guy. If the lug to lug length is not listed under the watches specs just do a google search.

A few other things - don’t buy from a jeweller or mall unless it’s discounted. Online sellers are often substantially cheaper. Either do a google search or ask on here for the best online seller in your area of the world.

And lastly, if it’s an option for you to have him choose the watch, do that. Watches are a very personal item and how they feel on the wrist is a big part of it - if possible, have him see the watch in person and try it on before you buy. You can do this in store and buy online obviously.

Hope this helps; good luck! ☻