What kind of body shape does he have? That's usually the most accurate way to tell whether they're over- or underweight, or normal. Overweight is round like a ball, and can have a hump of fat over their shoulders, and rolls of fat under their legs, in the armpit area. LizardGirl has described it as, if they feel "squishy", they're probably a bit overweight. Normal hedgehogs have either a teardrop shape (most hedgies) or look like a twinkie with straight sides | | (runner-type). Underweight, you'll see the sides cave in a bit when they're sitting there, like ) (. Sometimes runners can look underweight when they stretch out and it makes their sides look caved in.
I'm not sure if that's a fast weight loss or not...it'd be almost 200 grams in about two months, but that breaks down to about 25 grams a week, which doesn't sound so scary. Are you weighing him at the same time every time you do it, either before/after he poops? It doesn't seem like it'd make a difference, but hedgies have BIG poops and it can take off 10-20 grams sometimes if you weigh after they poop.
If he's eating and exercising normally, it sounds like he's pretty healthy now. I would definitely continue to keep an eye on his weight and see if it starts evening out soon, especially if he's starting to look more like the normal shapes for a hedgehog and not feel squishy. What food(s) are you feeding him, and what are the fat percentages? If his weight doesn't even out and it seems like he's going too far in the opposite direction, you may find that with a wheel and regular activity, he's turning out to be more of a runner hog and needs some higher fat in his diet.