Almost any pokémon can serve as an adequate starter for an eleven-year-old. It really depends on how patient the eleven-year-old is.
However, if you’re interested in giving him a water/fire/grass/electric-type and if he’s very interested in training a ghost or dragon, might I suggest rowlet? They’re traditional starters in Alola (and thus easier to care for than most), but they evolve into the ghost-type decidueye.
Alternatively, the water/ghost frillish, the grass/ghost pumpkaboo, and the fire/ghost litwick aren’t that difficult to take care of. They’re simply a bit on the mischievous side, especially litwick, and may require a bit of extra patience and awareness. As for dragons, there is technically horsea (which evolves into the half-dragon kingdra), but seeing as that evolution is induced via trading or extreme age, it may not suit your cousin’s tastes, and there aren’t really any other dragons that dual-wield elements from the list you’re considering.
Of these, though, I would still highly recommend rowlet, as rowlet are normally bred to be easier for beginning trainers to handle than most grass- or ghost-types. The only thing your cousin will need to worry about is rowlet’s nocturnal sleep cycle, but if he’s considering training ghost-types in general, he’ll likely invert his own sleep cycle eventually anyway.
Best of luck to both you and your cousin, anonymous!