I have just had blood test results back on one of my piggies - one of the liver enzyme result was off and apparently this is to be expected if the piggie has not been eating properly because of some other underlying condition. In my case the piggie had overgrown molars. It is a normal response from the liver.to starvation. If malnutirition continues for a length of time, then a condition known as fatty liver can result but at this stage with Thorn, once he starts eating properly and regaining his weight there shouldbn't be any further problem
However, you mention a high white call count/infection - did the results say whether the white cells were neutrophils, lymphocytes oir eosinophils? This gives an indication of the type of infection (Bacterial, viral or parasitic respectively) or whether there is just a general anti-inflammatory response.
Some infections do reach the liver (it is a major defence at filtering certain nasty bugs from the blood) so the liver enzyme results could also point to a systemic low grade infection.
Hopefully the septrin will clear the infection as it is a very broad spectrum antibiotic and active against most of the more common bacterial infections of guinea pigs (unless the bugs have drug-resistance).
I'm surprised the vet has withdrawn metacam on the basis of liver issues because metacam is generally regarded as having renal toxicity issues, especially in animals already renally challenged.
If pain relief is required to get Thorn eating more. perhaps the vet would prescribe tramadol instead. (My vet uses this in preference to metacam now for pure pain relief if anit-inlalmmatory intervention is not required - which is what metacam is best at) .
As to diet - plenty of fibre (some nice hays perk them up), extra critical care or mushed pellets in between, extra vit C, veggies (if his gut is OK) and dried herbage to include echinacea (coneflower) for the immune system and milk thistle for the liver. The Woodlands guinea pig herbal forage contains birch leaves (for urinary issues) willow bark (care here as willow contains aspirin which is classed as non-steroidal anti-inflammatory) and echinacea. Dried milk thistle can be obtained from Galens Garden or you could discuss with the vet using a capsule supplement and sprinking the powder into the critical care.
HTH and he makes a full recovery shortly
x.