I would either suspect a spine injury or scurvy (vitamin C deficiency). Please have him seen by a good vet as soon as you can. Your boy may need a scan to check for a spine injury.
Recommended piggy savvy UK vets:
Guinea Pig Vet Locator
Recommended vets in other countries:
Veterinarians - The GLX-Files
Please weigh daily at the same time to monitor the food intake and if necessary start topping him up with syringe food and water. Place hay (which should make up to 80% of the daily food intake) next to where he is, so he can still eat. Start syringe feeding as soon as he has lost 50g or more. it is vital that you keep the guts going and balanced. You can five 1/8 of a human vitamin C tablet once or twice daily, if necessary dissolved in 1 ml of water and given in 3-4 mouthfuls to prevent it going down the wrong way in such a little one.
Keep the area where he is sitting dry and cleaned twice daily. Can he clean himself? If not, please give him a gentle wipe with a damp cloth or unscented puppy wipe. Be careful when handling/lifting him in case he has got a spinal injury.
Here is our illustrated step-by-step syringe feeding guide with tips on what you can do with what you have at home or easily available:
Complete Syringe Feeding Guide
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