I need something I can add a grip too really
Eh?
I have banana fists.... despite my D3200 being one of the more compact DSLR's about, I have absolutely no problem with it's size, let alone compunction to add a pseudo-winder to it to make it 'easier' to handle. It's actually physically bulkier than the Olympus OM film cameras it superseded, which in turn are bulkier than my most used Olympus XA2 compact, which curiously isn't a heck of a lot larger than the pocket digitals that I have been using for the last decade and a half... that are themselves considerably bulkier than my Minox C 'James-Bond' Spy camera!
The Notion, I am afraid, just boggles....
If banana-fists come into the equation at all with more modern gear, it's that they have such diddy buttons, or no bludy buttons at all!
My Daughter's an smart-phone junky; "Just touch the icon on screen Dad!" err... yeah... it's showing about twenty of them, and my little finger's covering four of them!
O/H's son, handed me his rather much larger , I think Samsung 'something' swiss-army-communications-console with added under-chin lighting feature, that's about the size of a lap-top... without buttons! Y-E-A-H... and this helps 'how'?!?
The Fuji, AFAICT, aping 'retro-styling' of old mechanical film cameras. is significantly lever, dial and wheel 'driven' rather than button-set, which may 'help', but, this is the nub, it's rarely the physical size of the camera, but the buttons that matter, and I think you are possibly diving down an cul-de-sac chasing technology to solve 'problems' it has made! And I suspect that switching systems on this nebulouse notion, may make even more niggles for you!
Son's Situation? He's 12 you say? What's the likelihood he'll elect to do GCSE photo at school? What's the equipment list for GCSE Photo at his school? Is the tutor a Nikon or Cannon person? This would be steering my choices as far as what would best suit the lad, and I would likely be looking for something 2nd hand, at the older end of the entry level range for him, for least cost/risk starter set up, to get him going and familiar with the same conventions as he'll be using at school if he goes that way, and scope to upgrade as ad when needed.
Daughter dd her GCSE and A-Level photo courses, and as a Nikon user, we bucked the school-script getting her a D3100, as she was already familiar with my D3200 and 'share' my.. lenses which, proved a much smaller cause of controversy than the batteries, they also shared... y-e-r-s... I always know which ones are 'mine'... they are the flat ones!!!!
That 'experience' does beg some suggestion that the 'idea' of compatible systems is something of a double-edged sword... Her tutor was a Canon-Man, she was one of just two Nikon shooters on her course, and spent more time teaching the other Nikon user to do what teacher had done with a Cannon... and arguing with me, when she had to ask, as teacher didn't have an answer.... and the whole 'issue' of shared lenses was a very big blue touch-paper.... of course the lens she 'needed' was always the one on the front of my camera... and when she started dragging them through the bushes in the local park, or chucking water filled baloons around 3" from their delicate electronics, and grumbling that she 'had' to take them to school, but had no-where to keep them..... It WAS all a lot more hassle than it was worth really.......
I really believe that there is little advantage to sharing a system, and a lot of potential disadvantage, room for controversy and consternation. And ultimately, I ended up handing her the bag and telling her to get on with it, and shooting a film camera!
SO! separating the variables here....
First, I don't think that there is an awful lot of useful advantage in sharing a system with your son.
Next, selecting system for him, possibility of school-photo-courses has to be considered.
Then, sorting out 'son' should be 'one' job to be sorted, independent of anything else.
THEN, you can ponder your problem and needs, as an entirely separate topic. In which the handling of the Fuji vs anything else should be weighed the pro's ad cons, very carefully. Merits of mirror-less, systems, do, to me pall in comparison to the advantage you have pointed out of DSLR systems commonality and consequently better, cheaper availability of lenses and other accessories... but you only need one lens to take a photo.. ad the 18-55 kit o my D3200 has consistently been my most used lens over the last five years, despite the £1000's worth of alternatives I have stuck in the bag desperate to get the same sort of 'range' I had for film!
And 16mm to 200mm? That is a pretty useful range of focal lengths; other than the fiddly-fingers issue dealt with at start, what 'really' does the camera not do for you? Would anything else 'really' do the job much if any better or cheaper?
As said I think that the idea that a DSLR and battery grip will be 'easier' to handle, may be a little bit of a red-herring; and the idea that a battery grip, would help, is rather nebulous. Clue is in the name, a battery grip is there to take extra batteries so you don't have to swap them so often, when they go flat! Is that a problem you often encounter? As said the side-effect of added grip-space has to be countered with how much is the actual 'size' and how much 'buttons'... would it really 'solve' a problem or just swap the problems you have?
If you aren't getting on with the Fuji, then, yeah, little reason to stick with it. B-U-T, in the bag of issues you suggest you want to tackle, dont chuck the baby out with the bath-water!