I think watches are very personal. I've fallen in love with a watch in a shop window on several occasions, only to take an instant dislike to it when I try it on.
In other words, @Snooks1971 rather than surprise your son, it would be best if he sees and tries on a few watches to see what feels right on his wrist.
And, without wishing to sound unkind, if you've never heard of brands such as Longines, Skagen and Mondaine, then I'm guessing that you're really new to the watch market.
So I'd go to a specialist watch dealer as opposed to a high street jeweller. If you can tell us whereabouts in the country you are, someone will be able to suggest somewhere. (If you're anywhere near the west Midlands, I'll put in a shout for
https://www.firstclasswatches.co.uk/ in Kenilworth. It has watches for all budgets! No high pressure sales pitch, and they will usually knock a few ££ off if you sweettalk them!) And if your son falls in love with one that you can't afford, you could always have a look at the used watch market at that point.
(I am not a watch expert either! But between us, my OH and I at the moment have omega, longines, maurice lacroix, skagen, bering, rotary, tissot, tag heuer, and I'm sure there may be others that slip my mind!)