I had a guinea pig that itched her paw every time you scratched a certain place on her back. XD
It's a silly nerve connection thing, most likely. Like if you have a place on your leg that you scratch and a spot on your arm randomly tickles. He's fine.
But if it seems to bother him, I'd keep from scratching it too much.
Now, if he shakes and purrs, that's something else. Mine was a female, so I know the purring meant either one of two things: "WHAT THE HECK WAS THAT?" or "Ooh la la~" XD
It's hard to tell if he likes it or not though if he purrs, since he's a male and I don't know much about males, but I'd say again: Just refrain from doing it if it seems to bother him. You'll know he's especially bothered if he suddenly jumps and squeaks. That means "ALRIGHT ENOUGH GAWD."
If your scratching his lower back....youre making him horney.
Im being serious. My guinea pig only does this when I scratch her there
Well, sometimes, when guinea pigs shake, they're actually making a purring noise.
But, your guinea pig could also be scared of you, since they shake when they're scared too.
Also, if she/he is squeaking, thats probably fine, Your guinea pig is probably just scared. But if its a loud, high pitch, squeaking noise, thats diffrent. She might have broken a bone or have some sort of problem.
But if she/he is making a normal, squeaking noise, then your guinea pig probably isn't hurt.
If she/he ( Urghh i'm just gonna call it a he okay? XD ) if HE is shaking, then he is probably scared of you, or cold. If he's cold, wrap him in a blanket.
If he's scared, try to get him more used to you, pet him every-day, E.T.C