We need to know the exact amount of weight he has lost and over what time period.
Poop putout is 1-2 days behind food intake so by the time you see less poop they already haven’t been eating for a lot longer than you think (which is why the routine weight checks are so important).
If you’ve only been able to tell how much he’s lost through touching him ie you haven’t been weighing, then the amount of loss is probably more than you think.
A change in behaviour is also of concern.
Weight loss is considered loss when they lose 50g or more.
100g of loss in a day or so is considered an emergency and will not wait.
This is why knowing the exact amount is important.
From what you’ve said - lack of poop, weight loss, trying to eat but can’t - he needs to be syringe feed a recovery feed or mushed pellets. You will need to increase the weight checks to daily. Weigh him on kitchen scales every morning. Syringe feed as much as is necessary in a day to keep his weight stable - this is a minimum of 40-60ml each day but he could need as much as 100ml. It can mean syringe feeding every two hours.
Of course, he needs to see the vet so it’s good you’ve been able to get an appointment.
The guides below explains everything further
Emergency and Crisis Care as well as Bridging Care until a Vet Appointment
Not Eating, Weight Loss And The Importance Of Syringe Feeding Fibre
All About Syringe Feeding and Medicating Guinea Pigs with Videos and Pictures
You say he’s a menace and torments the other pigs.
How many other piggies does he live with?
What sexes are the other pigs?
How old are they?
I'm just wondering (through your use of the plural ’pigs’) if you are trying to keep more than two boys together. If so, then that may be an issue and why you describe him as a menace and tormenting the others. Boars can only be kept in pairs and any more than two together usually fails requiring the piggies to be separated.