Why does my 9 week old guinea pig LEAP all over his cage?!

[Alambil]

He's terrifying the other pig!

I've not had guinea pigs, but have had baby rats/mice/hmasters etc as a child teen and used to hop and bounce everywhere till they were bigger.

If he's bouncing madly into the air it means he's happy (they call it popcorning), so you're doing well!