Maybe you should tell us what
kind of crappy iPad games he plays? Not giving him stuff he doesn't like is important. Personally when I tried to get my nephew onto the good shit I gave him Battle Squadron (
https://en.wikipedia.org/wiki/Battle_Squadron) because it has fairly nice graphics and he wasn't allowed to play 'shooting games', as he called them, at all (I was trying to position myself as the cool uncle). Last time I saw him -- we live in different countries -- he was mostly playing consoletrash FPSes, Minecraft, and tryhard Minecraft clones. I have disowned him, of course.
In any case, I probably wouldn't give him any RTSes or any other strategy games really, unless that's what he's used to. If you've ever seen the Lego Movie you'll know that this is the epilepsy generation and anything too slow or complicated is going to be too far outside their range of experience for them to enjoy much. Anecdotally I'll say that my nephew just about shat his pants from boredom when me and my brother intro'd him to pen and paper D&D. As such you'll want something with a short learning curve and immediate fun, like a decent platformer. When I was that age I liked Ninja Gaiden (Sega Master System version), Prince of Persia and Flashback best, although I am a product of my time. Earthworm Jim and Jazz 2 are good suggestions, but I would recommend letting him use a gamepad, especially if he hasn't used a keyboard in any serious way before.
Chances are good he's also tolerant of building things/crafting (Minecraft is on iPad so far as I know, so I assume he's already into that. They all are), so anything with simple SimCity-type gameplay might appeal. I found SimCity 1989 manageable at that age but I didn't like it much, and to this day it isn't a genre I enjoy so I can't really recommend anything.
Re: adventure games, I'd skip Pyjama Sam. I used to work at a library and that shit is for 5 year olds (based on the fact we had computers with it installed and the 5 year olds gravitated to it, and also the plots are about being scared of the dark/lightning and so on, ie. very young children things. The older kids preferred the Wii/XBox/PS3). Monkey Island has plenty of kid-friendly jokes and nice visuals so just give him that. Surely there's a ScummVM implementation for iPad too, so the interface will be very friendly for him. FWIW Loom was too weird and easy for me as a kid but I think I played it when I was a bit older, about 11, so an 8-year old might find it about right.
Also, as long as I'm talking about my old job, Minecraft, Lego Whatevers, and pretty much anything on the Wii were very popular among kids that age. Age stratification went something like: preschool-6 years, Pyjama Sam and sometimes the Wii, 7-10 Wii, 11-15 XBox/PS3, although the waters muddied a bit when it came to Lego Whatever and Minecraft, as mentioned.
I'm at the point now where I'm rambling, so this is the end of my post.