Please have him seen by a vet for diagnosis and treatment.
Switch from the routine weekly weight checks and instead weigh him daily (at the same time each day). This will enable you to more closely monitor his hay intake. Hay is their main food source but you cannot judge their intake by eye - weighing them is the only way to know they are eating enough (veg and pellets are such a small proportion of their diet that seeing them eating those isnt enough). If he is feeling unwell and isnt really leaving his house, then its possible his hay intake may reduce - ensure you put plenty of hay into his house so he can eat without having to come out if he doesnt want to.
If you see weight loss then that means he isnt eating enough hay and you will need to step in and syringe feed him.
Emergency, Crisis and Bridging Care until a Vet Appointment
Not Eating, Weight Loss And The Importance Of Syringe Feeding Fibre
Complete Syringe Feeding Guide
Weight - Monitoring and Management
As we cannot see what kind of symptoms your piggy is showing, I am also attaching our ringworm guide to provide you with further information in case it is the infection he is experiencing.
Ringworm is a highly infectious fungal infection and needs to be treated properly including rigorous hygiene of the cage, equipment and yourself from the beginning to stop it from spreading.
Ringworm: Hygiene, Care And Pictures
Keep your piggy in with his cage mate - dont be tempted to separate them. Exposure would already have occurred and separating them can cause stress