Glad you got a scale. Keep weighing him on a regular basis to track his weight. Unexpected increases followed by significant declines are often a great indicator of illness (ie, feeling a little off so still eating, but not running around and weight goes up a little followed by feeling worse and not eating as much and weight drops off)... as are just plain declines that aren't accounted for by anything else (like change in diet, newer/better wheel, etc...).
Yep, like charlie (April) said, there's a fair amount of variation among hedgehogs. It's kind of like humans that way; eg, 100 pounds on a 5' tall person is okay, on a 4' person means they're obese, and on a 6' person probably means they're near death from starvation.
So, take a look at Sonic from the top when he's standing normally (ie, not all puffed out in a quill ball or stretched out in Superman pose). Draw two imaginary parallel lines from his front to back hips. If his sides roughly follow those lines, he's in a good range; if they sink in like an hourglass, he's probably underweight; if they bulge out like a football, he's overweight.
One thing that might trip you up on that analysis is if he's one of the more teardrop shaped hedgies. Some are just more naturally shaped like a teardrop with bigger hineys than their front ends. You can do the same kind of comparison as above -- do your best to apply it to a wedge shape instead of using two parallel lines.
Also, take a look at his legs, if they're all blobby/smushy at the top when they join his body (like ham hocks), he could probably stand to lose a bit of weight.
That said, I don't mind if they're a little overweight. When they get sick, they tend to lose weight quickly when not eating, so if there's a little (not huge) cushion to absorb that, I tend to think that's okay. Others may be of different opinion.
With those ideas (including charlie's) in mind, how's he looking?
