I’m sorry to hear he is unwell.
Metacam tastes nice so most piggies take it willingly. Baytril, on the other hand, tastes disgusting so he is not going to be willing to take it. It is better to syringe meds directly (so you can be sure they are getting the full dose so well done for being able to do that) and then following it up with a tasty treat. He may now associate the nasty taste of baytril and veg. It is going to take a while for him to trust veggies again without baytril being on them.
Being off his food - so hay is their main food intake and is 80% of what he needs to eat in a day. You can only gauge hay intake by weighing piggies (you can’t know they are eating enough by watching them as it is too deceptive). Piggies need to be weighed weekly as part of routine care but when they are unwell or if you have concerns, then you switch to weighing them daily (at the same time each day).
If he is eating hay and his weight remains stable then he is not technically off his food at this point.
If it is just the veg he isn’t eating, then as above, it may simply be because of the baytril association.
If he is not eating enough hay either and his weight is dropping, then that is a different thing. This can happen when they are unwell.
This means he needs to be syringe fed a recovery feed to replace the hay intake (you can mush his normal pellets with water and use that as a syringe feed if you don’t have a proper recovery feed). This is essential and literally lif saving. You need to feed as much as is necessary in each 24 hour period to keep his weight stable, and this also depends on how much hay they are still eating independently even if it is a reduced amount.
Also with antibiotics, it can affect their appetite and consequently hay intake. The antibiotics can kill off the good gut bacteria and make them feel off, Again if this happen then he needs to be syringe fed.
It is a good idea to give probiotics to a piggy on antibiotics as it can help settle their gut. These need to be given either 1-2 hours either before or after an antibiotic does (never at the same time as). Better still, if he has a companion, then poop soup is a great way to boost the gut of a poorly piggy.
Weight - Monitoring and Management
All About Syringe Feeding and Medicating Guinea Pigs with Videos and Pictures
Probiotics, Recovery Foods And Vitamin C: Overview With Product Links
The sediment - depends very much on whether he can flush this out himself. Make sure he has lots of water and watery veg (may be tricky now with the baytril issue) as this can help him pass whatever is in there. Sometimes though a bladder flush may be needed.
It’s a good idea to ensure his diet is now as good as it can be.
High calcium intake and poor water intake can lead to this kind of issue and stone formation. It’s not something you’ve done as these issues take a while to form but it’s good idea now to make sure that his diet is kept in check.
Ensure most of his diet is hay. He can have one cup of veg and just one tablespoon of pellets per day.
Make sure high calcium veggies such as kale, spinach and parsley are kept very limited - no more than one small amount once a week. Also ensure carrots and fruit are kept out of the diet or kept as an occasional treat.
Ensure his dinking water is filtered to help keep calcium intake down. Most calcium comes into the diet via unfiltered drinking water and their pellets (even low calcium pellets contain a lot of it!)
Long Term Balanced General And Special Needs Guinea Pig Diets