Honestly, any RPG--particularly on primarily 2D platforms like the GBA--would probably be a good place to start. Outside of Pokémon, Mario & Luigi: Superstar Saga is also a very good and child-friendly RPG. I don't know how old your cousin is, but if he's on the older side, then you might introduce him the GBA remakes of Final Fantasy IV and VI. Maybe. Outside of RPG games, pretty much any Zelda game will require a great deal of reading in order to progress and would probably hold a child's attention slightly longer. The GBA port of Yoshi's Island is also pretty text heavy from what I recall. The Sonic Advance games have cutscenes with dialogue (though I'm hesitant to recommend them because they auto-skip, even I sometimes struggle to finish reading before they move on).
Metroidvanias might also be worth looking into. The namesakes of the genre (Castlevania and Metroid) might be good to start with, sans the GBA port of the first Castlevania game which is more linear. The Mega Man Zero games are also fairly plot heavy and require extensive reading to understand, but be warned that they are difficult. I'm not an expert on the Harvest Moon series, but I would imagine those would have to be pretty heavy with text as well, given my experience with similar games like Stardew Valley.
I'm sure there's more that I could think of if I racked my brain hard enough, but this should give you some ideas. Though to be honest, I'm rather skeptical of the idea of getting them games that will force them to read. You know your own family better than I do obviously, but unless you plan on playing these games with him (rather than merely gifting him the games and leaving him to his own devices), I can very easily imagine him skipping the dialogue and tutorial text when at all possible, and giving up when he hits a brick wall. I'm not saying you shouldn't give him these games, as they're all great games, but (and I stress, I am not an educational expert or anything of the sort; just someone who has seen this happen in their own family) I'd advise to supplement it with other activities that foster a love of reading, i.e. group activities, etc.