Rematching gym leaders is a great way to get EXP. since he's pretty underleveled, you might want to use an EXP share, or use a really strong pokemon. Battling Red will also get you lots of EXP. If you have the item Lucky EGG, that will give you 1.5 more EXP. Mt. Silver is a good place to train due to the powerful wild pokemon. Trading Shelgon will also make him level up faster, because traded pokemon yield more EXP. After training him, you can just bring him back to your main game.