Please have him vet checked. That is a massive weight loss that needs investigating! 50g/2 oz is a weight swing that is within the tolerance zone; any more is not normal.
Please start syringe feeding asap to make sure that your piggy is OK. Please also keep in mind that you cannot see the crucial teeth at the back of the mouth (overgrowing roots or dental abscesses) nor any internal lumps or problems. In most cases, a pain issue is at the bottom of any major weight loss. Please also take into account that hay is making aroun 80% of th daily food intake, but it is usually the food group that you cannot control and that is the first to be dropped.
Weigh a healthy piggy once weekly and a piggy with concerns once daily at the same time in the feeding cycle (i.e. before breakfast or dinner) to monitor the food intake. Cheap supermarket kitchen scales will do.
Complete Syringe Feeding Guide
How To Pick Up And Weigh Your Guinea Pig
The Importance Of Weighing - Ideal Weight / Overweight / Underweight
here is a list of recommended piggy savvy vets in Ontario:
Veterinarians: Canada - Guinea Lynx Records