Hi!
Please have him vet checked. This amount of hair loss needs proper investigating.
Have you been weighing your boy weekly and noticed a weight loss?
It could be mange mites, fungal or self-barbering from a pain issue in his lower body (to me, it looks like the most likely issue but I can only guess and not diagnose). Barbering from a companion in this location would be unusual in this location.
Scurvy in well kept guinea pigs on a hay based diet with vitamin C rich veg and herbs and vitamin C reinforced pellets is very unusual, unless your piggies have been on extra high vitamin C for a long time; in the latter case, a sudden illness related drop in very high vitamin C levels can result in scurvy symptoms (like additional hopping etc) even though the actual vitamin C level is still higher than normal - the problem would be finding the underlying problem.
Irregular hair loss in combination with a quick weight loss can be in very rare cases a symptom of Cushing's disease. This is generally fatal in guinea pigs.
Please do not home-treat on spec. You can easily make things worse than better if you get the wrong thing and you make a vet diagnosis a whole lot more difficult.
Guinea Lynx :: Hair Loss
Barbering ( Eating Hair)
New piggy problems: URI - ringworm - skin parasites