His right hand (leading hand) is fully functional but his left hand is 'weak he can move it but he has no power. I could imagine he could lift a pen if his arm were rested but to lift his full arm and use the muscles in his hand I don't think so.
Everything will be based around his right hand so he has the power to plug in the charger and to type on the keyboard I'm getting him.
Sorry to hear about your father, has he managed to gain strength since the stroke? Are people able to gain strength after a stroke or does it just happen and then that's the end of that?
You've got a lot more mobility counting in your favour there and I can honestly recommend rather go for a laptop that can be placed down anywhere and everywhere and do all the functions you require probably for much less money and much less complexity all in one unit.
You could still optionally do the shelf on the wheelchair, but like I said the odds of it being used frequently will happen less and less as time goes by.
If anything you could maybe consider one of those transformer type tablet/laptop hybrid things but even that might be too complex for his needs. And if you are worried about Windows being too hectic for him to operate then either lock it down with a Standard account that he cannot break...or go the route of installing one of the very simplistic Linux distributions like ElementaryOS.
Sadly my dad has done nothing but decline since he first came home about five years ago. When he first got out of rehab he was able to lift a glass and somewhat feed himself for short bursts of time and at one point he was even able to lift his legs slightly from a sitting position.
However maintaining that and increasing it takes extreme amounts of upkeep and is simply not plausible in most cases.
So at preset he can pretty much operate a trackpad on his laptop and also the keyboard but he finds it easier to type with a stick in his mouth instead most of the time.
Therefore as hard as it is I would recommend you think ahead five years and plan for the fact that your grandfather will more than likely decline drastically in that time. And iPad required a massive range of movement to operate, something us able-bodied people take for granted as it comes so naturally to us.