my 2p
Also consider that if he doesn't already have software compatible with the drawing pad he won't get much benefit - you really need drawing software that handles layers. ( & If you go the tablet route then you'll also need a pencil or equivalent. )
Last year DD wanted a tablet and ended up (GP supported thankfully) with a HUION Kamvas Pro 13 - www.amazon.co.uk/Display-Drawing-Function-Battery-free-Pressure/dp/B07FC7W4Z4?tag=mumsnetforu03-21
Tablet was/is superb and after several months of cycling through multiple drawing programs she has settled on Clip Studio(£), with some specific use of IbisPaint(free). All supported by KineMaster and VideoStar because her current main focus is producing short gacha animations for a uTube channel.
Autodesk Sketchbook is actually free and an excellent program to try and help determine whether you need other capabilities.
Clip Studio Paint Pro @ £40 is superb for everything except animations - madam ran up against the limits in this after a couple of months, we had to go for the EX version in lockdown - ££££ :(
In that year her use of the tablet has changed from 100% to ~10% and she has reverted to using the same programs on an iPad.
Subjectively it feels like the tablet let her gain skills with the drawing programs such that it became possible to use them effectively on the tablet.
Good drawing programs are very complex - even use of core techniques like pinch zoom in/out, canvas rotation and brush/colour selection take quite a bit of practice and along with layers are vital to getting a good result out of the tools - once you've learnt those techniques then you can use the programs easily on pad or tablet.
There are oodles of tutorials, in great depth, for the various tools on the web. Photoshop & the adobe tools used to be the ''best' - IMHO they're far too expensive relative to the contenders and these days are arguably less capable in the things you want to do - because they are overwhelming and split capabilities across tools.
In an ideal world we'd have hired the tablet for a year and updated her iPad for one with more memory - I don't think that is an option - if it was I'd go for that. As it is her iPad update will have to wait.
If you want to see what is involved/possible with some of the tools have a look at "speedpaint" videos on utube.