BIG HUGS
I am sorry about the bad news. Sadly there is not much in the way of treatment for liver problems in guinea pigs at this stage.
You may want to stay off veg high in vitamin A, like carrots, spinach and lettuce:
Guinea Lynx :: Nutrition Charts
Please step in with offering additional syringe feeding top-up support, whether that is by syringing or eating from a bowl if your boy is still doing that on his own.
Complete Syringe Feeding Guide
If you have monetary concerns at any stage after having made an effort to have your piggy diagnosed, you are well within your rights do discuss whether any further expensive investatigations/medication is coming with a reasonable expectation of results.
I certainly do this with my own vets once we get into hundreds of pounds for any further investigations/treatments with comparatively little return. I am willing to pay (and have in some cases paid £1000-2000 in longer term treatment costs where long term prospects were good), but only if there is a positive expectation of any further treatment to heal or prolong quality of life for a good while. However, I do not have the resources of investing that kind of money for very little return in life expectancy and quality for every piggy of mine.
I did have this discussion with all of my regular vets fairly early on. They are fine with that and have adapted their treatment options accordingly.
Alternatively, discuss a payment plan if your vet is confident about any further investigestions and any longer term prospects.
It is a VERY tough and gutting time when you have to look at the hard facts and weigh up the pros and cons as to how far you want to pursue a more complex health issue and when you want to draw the line. Drawing the line and refusing to spend a fortune on a guinea pig where treatment options are limited doesn't make you a bad owner and you do not have to feel guilty about.
There is also a certain risk of putting your piggy through too much when you are afraid of letting it go or feel bad about having to make the decision to call it shots at some point - you will always have qualms about it. Walking the line is a tightrope act in which it is rather easy to fall down on either side of the rope.
It is one of the most difficult areas to work out your personal ethics. I hope that my own example can help you to address your issues with an open mind and to have an honest discussion about where you go from there with your vet. Any good vet will be prepared to give you the facts when asked to help you make an informed decision.
I have found that asking myself whether I can live with my decision in the long term of having found an acceptable balance between the amount/cost of treatment and providing quality of life in the longer term is my personal touchstone of working this out every time I am required to do so. But I have also not got any qualms about asking my vet how much time they think my piggy has got in their estimation and then take it from there.
I hope that this helps you.