Hi!
Baytril (as any other antibiotic) can completely wipe out the gut biome/kill the appetite in some piggies.
Please speak to your vet about whether it is better to push through or to try another antibiotic.
Make sure that you get at least 40-60 ml into him in 24 hours just to keep the gut going; ideal would be 60-90 ml but that can be too much of a goal with a piggy completely off its food. The poos are a sign that he is not eating and drinking as much as he should. The poo output usually lags a day behind the intake, but it can also be 2 days with a slow gut.
Does Theo have a healthy mate? In this case, you can support the gut with twice daily totally freshly made 'poo soup', i.e. the transfer of healthy live guinea pig gut biome; the less time between dropping the poo and syringe the water it has soaked in, the more live stuff will reach the gut. It is a bit gross but mimics natural recovery behaviour and provides tailor-made gut bacteria etc.
If done properly, it is more effective than probiotics. Give it either 1 hour before or 2 hours after the antibiotic.
In addition I would recommend a course of fibreplex which will add more stuff. Many piggies don't like having it squeezed into their mouth but will eat in their syringe feed.
You can also add some dioralyte to the water you are syringing if you or your vet a thinking that he may be a bit dehydrated.
Here is more information on how you can support Theo through this. Keep in mind that any overweight will come off very quickly in an ill piggy so the initial weight loss can be rather dramatic.
Emergency, Crisis and Bridging Care until a Vet Appointment
Complete Syringe Feeding Guide
Probiotics, Recovery Foods And Vitamin C: Overview With Product Links
All the best for getting through this!