a male eevee, lvl 22, holding a thunder stone.
I'm well aware that eevee becomes espeon with high friendship during daytime, but my he's holding a thunder stone so shouldn't he evolve into jolteon even during daytime? what am I missing here?
In order to evolve into Jolteon, you have to USE the Thunderstone. It doesn't require leveling up or holding it.
So, you need to take the stone back, and select "USE" and it should say Eevee is able. Then your Eevee should evolve into Jolteon.