Have the vets any idea what is causing it? What medication is he on? Has he got gut stimulants, painkillers and are you feeding enough? Have they checked for his gut movement, heart, airways, painful areas, any internal tumours, gas in the stomach etc.?
From feeding mushed veg instead of hay/fibre based recovery care (hay makes 80% of the daily food intake while too much vegetable can make guinea pigs ill), I assume that your vets are not very experienced with guinea pigs? if you can, please switch to a herbivore recovery formula like Critical Care or give mushed up pellets as an emergency stop gap measure.
You need to feed 40-60 ml over the course of 24 hours to keep a guinea pig alive. A healthy guinea pig in its prime eats about the equivalent of 120 ml in solids and liquids.
Here are our detailed illustrated tips:
Complete Syringe Feeding Guide
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