Checking his heft is putting your hands around his ribs to see if you can feel his ribs.
If he is truly overweight then you will not be able feel any ribs as they will be covered by an unhealthy layer of fat.
You should be able to feel his ribs. If you can then he is not overweight.
Please read this guide for information on checking heft.
Weight - Monitoring and Management
Weight loss is only considered loss when they lose 50g or more. Before that it is just normal fluctuation.
1ml of syringe feed is completely pointless! The absolute bare minumum to keep a piggy (who is not eating hay) alive is 40ml. We recommend a minimum of 60ml but could be as much as 100ml is needed (a healthy piggy would eat the equivalent of 100-120ml per day).
It’s those daily weight checks which tell you whether you are getting enough into them.
If you weigh daily and he weighed 1080 today, what was his weight each day for the previous few days?
Lack of poop suggests lack of food.
If he isn’t eating hay and isn’t maintaining his weight then do not stop critical care. He will go into stasis if you stop feeding him.
Not Eating, Weight Loss And The Importance Of Syringe Feeding Fibre
All About Syringe Feeding and Medicating Guinea Pigs with Videos and Pictures
For bloat, then veg needs to be removed from his diet for now. Bloat can come in waves - they can appear to get better and then go downhill again.
Medication needed to deal with bloat are painkillers and gut stimulants.
Please read this guide below
Digestive Disorders: Not Eating - Diarrhea - Bloat - GI Stasis (No Gut Movement)
When you say he was sick before and was on antibiotics, what was his illness?