Hello
I have had two poorly little piggies with on and off dental problem. Theo was ill for about 5months with his teeth.
Sometimes post dental treatment the mouth is really tender and once they get put off they get very scared of trying again thats why it takes so long for some to start eating properly again. So its firstly a matter of perseverence and getting the weight on and this requires dedication on your part, a piggie wont want to eat unless it feels better within itself.
For syringe feeding you can use the recovery type diets that vets offer but they are really expensive and your piggie has only had a dental problem so I would advise you buy a decent pellet food, or one your already using. Then add hot water,cover with cling film and leave to soak (some pellets will take longer than other- anything up to an hour) then use that to syringe, add more hot water to the consistancy you require. Serving it warm (not HOT) is yummier to piggies .
Using a pellet food makes it easier for the guinea pig to start eating normally as it has the same food. To the mushed up pellet I add probiotic, which you can buy at pets at home( add as directed) and a vitamin c liquid supplement (add as directed), also available at most pet shops). I give initially 2-5ml every two-three hours to restart the gut function properly. Please remeber guineas dont have our digestive system they need it to go constantly so leaving a guinea all night can defeat all the good work done during the day. A pig that is used to hand feeding will turn away when its had enough. Giving too much food at once can cause bloat and do them no good. Watch for a swollen belly. Little and often is the key. If no weight goes on....it's not enough try adding more into the feeds. My boys took 15ml some feeds but thats only my boys and they are big boys he he. It really is down to the individual pig so keep weighing.
Also a major important thing is tempt with soft fresh veggies cut up small to minimise chewing. Lettuce although not the best for them is my boys favourite and without it I would never have got them to eat again. Also try grass if you can get a hold of some fresh, free of pesticides and grass treatments. Also cabbage, tomatoes and cucumbers. Grass is probably the best all rounder. Once he was trying and eating quite a lot of the fresh stuff I added in celery and carrots.
Monitor his weight daily, it is important that its done at the same time and not straight after feeding.
If the piggie starts to try more himself reduce the hand feeding gradually, I went from every 2-3 hours to every 4 hours/5hours then 3 times a day to 2times a day and then not at all . Alongside increase the time between feeds, I lessen the amount of water added to the pellets, this makes it really hard to get in the syringe but it gets the piggie used to a thicker constistancy. Eventually I have to just use my pinky finger to scoop some up and place in their mouth, my piggies are extremely trusting and placid. But usually by this point they will be licking it off your finger.
Its really important you put in all the time in the next couple weeks as without it you'll end up with the same issue all over again as the teeth will overgrow.
Alvin was eating normally in 5 days, Theo took 3-4 months. This seems really long I know but each week there was small improvements.
And most importantly always get dental work done without a GA it makes the healing process a lot quicker and less stressful, no horrible GA to recover from and less time away from their cosy beds

Good luck X