I've played the game 14 times, but apparently forgot when you can catch or discover Latios :/
You can't encounter Latias in Pokémon Soul Silver. Instead, you encounter Latios.
When leaving the Pokémon fan club house in Vermilion city with Copycat's doll, you will meet Steven Stone. This will trigger Latios to start roaming Johto and Kanto.
Hope I helped!