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Is this something that has just happened? I ask this because it sounds like impaction but if it's very recent then it could be a digestive issues causing loose poops so it's more like a diarrhoea issue.
Is your pig okay in all other regards, acting normally and eating okay?
Impaction is not uncommon in older boars. Sadly it is not curable as it is caused by weakening muscles, but you can relieve some of the worst effects by a good fibre rich diet with unlimited hay; always feeding the same foods at the same time and regular cleaning of the sac - how often depends on the severity of the impaction.
Please see a vet in case there is an infection in the boar sac if the mass has sat there for too long. It is the redigested poos (caecotrophs) that get stuck, so you can offer the mass for your piggy to eat a bit; some do, some don't. Guinea pigs need to digest their food twice to get the full benefit from the comparatively low nutrient diet they have evolved on.
Impaction is unfortunately a very smelly and yucky job, but it needs to be done regularly to make sure that your boy is not in discomfort and he cannot get an infection in his sac.