Agree completely re flute and clarinet - they are nearly as popular as violin but there are only 2-3 of them in an orchestra compared to up to 40 violins.
The rarer instruments are: viola, double bass, oboe, bassoon, horn, trombone. If he plays any of those he is likely to have more opportunities in the future.
BUT it is also easy to move from say flute to bassoon, violin to viola, trumpet to horn, so if he is really drawn to one of the more popular instruments then there is no harm in starting one and keeping options open for a move later.
The most important thing is that your son enjoys the feel of the instrument physically and is drawn to the sound it makes. They are all very different so it’s worth trying out different ones and listening to lots and seeing what he likes.
From experience (former pro musician here) I’d say it’s easier to start woodwind and brass than strings (progress will be quicker), and (especially for woodwind, less so for horn) you don’t need as acute a sense of pitch as you do for strings.
It is very exciting - I think of it like the wand choosing process in Harry Potter! Let us know how you get on.