Sorry to hear about poor Pip. He does sound very poorly but it is possible for a piggy to recover from this kind of ailment.
Just wondering, is he only getting the grass/dandelion/water mix to eat? Have you tried syringe feeding anything else?
I'm no expert but I've been through a similar thing myself - my piggy was very ill with pneumonia and couldn't eat/drink for himself. Members here recommended that I syringe feed him either Critical Care/Science Recovery or a similar alternative. Perhaps you could give something like that a try?
If your Vet didn't give you any of the CC or Recovery powdered feed then don't worry as you can make a similar version which should also pass through a syringe well, using regular guinea pig nugget/pellet food (the plain brown stuff, rather than the muesli mix ). Burgess Excel Nuggets work very well, if you have those. Simply pour some just boiled water onto the nuggets and let it soak for a little while. When the nuggets soften you'll then be able to mush it all up with the back of a spoon until it's a smooth paste without lumps. When it's cool you can syringe feed it. I'd also hand feed some water by syringe, alongside the nugget paste, every couple of mouthfuls or so.
It's really hard, I know. If you're new to syringe feeding, then I'd say just make sure that you take your time over it, only giving a very small amount each mouthful (I keep it to approx 0.25ml max, each squirt) and make sure that each mouthful has been fully swallowed before you give the next.
The main concern is obviously making sure he eats/drinks but did your Vet mention probiotics at all? Lots of people use these during a course of antibiotics, especially Baytril. Perhaps something to consider but for now just hang in there with the feeds.
Good luck. Really hoping Pip responds well to the treatment and you see an improvement very soon x
