Please start weighing your piggy daily on the kitchen scales at the same time of day (the weight usually swings in 30g band over the course of the day) instead of the life-long weekly weigh in.
Hay is making 75-80% of the daily food intake and the silica in it is what is grinding down the molars and premolars at the back; the fast growth rate of guinea pig teeth has evolved specifically against the silica in grass and hat fibre. The premolars usually grow spurs with either bridge the tongue and eventually make swallowing very difficult to impossible or more rarely burrow themselves in the cheek.
Please read these these guide links here for much needed feeding support in order to prevent your piggy from slowly starving until they can be seen for by then much more expensive badly overgrown teeth.
Weight - Monitoring and Management
How To Pick Up And Weigh Your Guinea Pigs Safely
Not Eating, Weight Loss And The Importance Of Syringe Feeding Fibre
Emergency, Crisis and Bridging Care until a Vet Appointment (includes tips on what you can do in a pinch with easily available stuff until you can see a vet and until your piggy is able to eat).
How are the bottom teeth looking? Have they started pointing inwards?
Parental legal duties (see first chapter):
Children And Guinea Pigs - A Guide For Parents
Please remind your dad that the average healthy life span of a guinea pig is 5-7 years - and that in his youth, this was considered a normal life span for
dogs. I would also tell him that the longer he waits, the more any dental treatment will cost unless he wants to be responsible for deliberately starving a pet to death in household.