I've been through a couple of dental abscesses with pigs and have a couple questions.
-Did your vet leave the wound open to drain after lancing the abscess? Did he give you instructions on cleaning the abscess if it was left open? Basically, abscesses will continue to refill and need to be left open to continually drain. It will help if you use a syringe of sterile saline or a cleaning solution from the vet a couple of times a day and gently express out any pus or discharge. It looks unpleasant but unless the abscess capsule can be removed completely, keeping the wound open and aftercare are really important.
-Did you vet give your piggie any antibiotics? Any pain relief? Although guinea pigs have really thick pus and an antibiotic won't be enough to clear up on abscess all on its own, it will help if it's used along with lancing/draining and cleaning. Likewise, pain relief like metacam may be needed. This is probably pretty painful and guinea pigs are often not good at dealing with pain. It's important to give pain relief so that chewing isn't so uncomfortable that the piggie just won't chew.
-For syringe feeding, how much are you giving and how often. Is he wanting to eat the syringe food or is he fighting you on it? If he's losing weight quickly, he is probably not taking enough in despite your best efforts. Wiebke posted a really good guide for syringe-feeding above so give that a read to figure out how much he's getting and whether it should ideally be more.
Definitely see the vet again, ask about some of these things, and see what they say. Does this vet regularly treat guinea pigs and their dental issues, because sometimes a vet with more limited experience will not cover all the bases as far as antibiotics, aftercare, etc. But abscesses generally are not always an easy fix and can take a lot of time. If your vet is limited in their knowledge about this, it might be worth seeing a more experienced vet as a second opinion, as guinea pig teeth can be really finicky and the more experience the better.
Hang in there and hoping things turn around for you and your piggie!