Yes he will need a cellular watch and an additional cellular plan. It’s a standard £5 so far from every supported network - some utilise your plan mins/data and others have their own allowance or are unlimited.
I think EE is unlimited mins and 5GB data on the watch for £5 a month
[pedant]
There’s no such thing as an Apple Watch. At least get the device name right.[/pedant]
I pay EE an extra fiver and get unlimited calls from my Apple Watch plus 10Gb data. Most months I use zero but it’s worth it being there for the handful of times I’ve needed to use it.
It's because Swatch successfully opposed Apple's "Apple Watch" trade mark application in the UK:
https://www.ipo.gov.uk/t-challenge-decision-results/o30716.pdf