Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'd like to help with your concerns about your fellow's lethargy, and lack of an appetite for the past two days.
You are right to be concerned as a guinea pig that doesn't eat well is in big trouble. Their gastrointestinal system needs to have food constantly moving through it or normal motility will be lost and there will be overgrowth of bad bacteria.
If he is refusing to drink water as well that is very worrisome too as they can quickly become dehydrated. I suspect his lethargy is a direct result of him not eating and drinking and a reflection of how sick he is.
It isn't possible for me to know what started all of this without physically examining him but possible causes include vitamin C deficiency, which leads to painful joints and poor mucous membrane health which can make eating and chewing painful, a respiratory infection, overgrown teeth or spurs on the teeth, especially the premolars and molars which can develop painful spurs over time that make eating painful, or metabolic diseases like kidney disease or diabetes, or even cancer.
But my concern right now is also with his gut. He needs to eat and he especially needs fiber to get his intestines moving properly. Make sure to have fresh timothy or mixed grass hay available to him at all times.
Because he won't eat on his own I highly recommend force feeding him.
You can use a medicine syringe and feed him ground up pellets mixed with water or orange juice or vegetable baby foods (carrots squash and green beans seem to work best). If you can find it Oxbow critical care for guinea pigs is even better as it is a balanced diet that can be force fed. Oxbow critical care can be purchased through your veterinarian or some petstores may carry it. You will need to force feed every couple hours during the time he's awake.
I also recommend getting him some probiotics to replace the good bacteria.
If it is cold where you are put him in a travel cage with lots of bedding to snuggle in or place a heating pad under part of his cage set on low to give him warmth. We don't want him wasting too many calories just trying to keep warm.
Then do your best to have him seen by a veterinarian that is familiar with guinea pigs. I'm afraid finding one today on a weekend may be very difficult. Here is a link to help you find a guinea pig veterinarian: http://www.guinealynx.info/vetlist.html
If there isn't one close to you then call local veterinarians and ask about their experience with guinea pigs.
Here is a link that will give you tons of information about guinea pigs: http://www.guinealynx.info/index.html
Please let me know if you have any further questions.