Hi there. Sounds like you have maxed out your effort values on that Sceptile.
You can check this by speaking to the girl beside the Energy Guru in Slateport city. If you are maxed out, she will give your Pokémon an effort ribbon.
Don't fret too much, though, because you can use berries to adjust your stats a bit.
The Hondew Berry will reduce your Special Attack EVs, which makes it a logical starting point. You can check the rest of the EV berries here in the Gen III section.