Please try to help stabilise the gut with 'poo soup' from healthy piggies (i.e. transfer of live health cavy microbiome). If made properly, this is the most effective method of helping a gut with dysbiosis (overgrowth of the wrong bacteria). If you use fibreplex, then please do a whole course or even two courses. Keep your piggy off any fresh food until the poos have been firm for at least 48 hours and then re-introduce it only very slowly, starting with fresh herbs and not adding any more vareties until the last lot has been fine.
Our 'poo soup' recipe is in here:
Probiotics, Recovery Foods And Vitamin C: Overview With Product Links
If problems remain ongoing, please see a vet. Sensitivities can develop in order older piggies, and as yours is a skinny, he is now an older gent.
Digestive Disorders: Diarrhea - Bloat - GI Stasis (No Gut Movement) And Not Eating
Long Term Balanced General And Special Needs Guinea Pig Diets (see special needs diet tips in the case of sensitivities)