HiCustomer Thank you for asking your question on Just Answer. By the way, it would help us to know:
How long has this been going on?
Does he live alone?
Has he lost weight?
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I would be very worried that this guinea-pig could be going the same way as the friend, sadly. I would be most concerned that you have seen a reduction in the feces production. It makes me think there is a gut stasis problem and possibly a bit of gassy bloat too. It is very painful for them and stops them wanting to move about at all. If it does not improve it will usually be fatal.
It may be best to have a vet confirm this is the problem, if you want the best chance of saving this pig. In the meantime, you need to syringe feed as much as possible. Dehydration is a big risk so making sure you are giving plenty of water and also liquidised food. There are special recovery solution available for rabbits and g-pigs but if you do not have access to these today try syringing vegetable pureed baby food. This is second best but better than nothing.
I do hope this helps your little pig.