Thanks for the responses so far.
I've got 1998 rigid Stumpjumper and I love it. All my bikes are in steel, vintage and new, and that's my preference.
I've offered to buy my son a bike and that's why I'm doing my homework.
He's had a rigid mountain bike in the past and he prefers straight bars.
He actually requested a new steel rigid mountain bike without realizing that they are no longer made.
There's no rush.
I'm thinking of something good, 21 inch, steel, responsive, 8 or 9 speed and can take a rear rack. A bike which if he takes care of it will last him a lifetime.
Like me he wouldn't use if for mountain biking but for riding on lanes, roads and tracks.
I'm not taken with modern hybrid bikes. They look flimsy to me and having front suspension forks on some of them is ridiculous IMO.
I've heard and read stories of catastrophic frame failure.
Another possibility is a straight bar steel tourer such as that offered by Spa Cycles, Hewitt or the Oxford Bike Works.
The latter's bikes have a "restrained" mountain bike look to them which is interesting.
However a new tourer with mountain bike gearing would be about £1500 and upwards whereas a good vintage steel mountain bike would probably be less than a third of that.
I have vintage and space frame Moultons and a Hewitt tourer. I ride them all but the bike I ride most is my rigid Stumpjumper.