Oh, poor little guy! Keep on trying to feed him, especially if he still has some pep to him and will take the food. My experience is that when there's no point any longer to syringe feeding, you'll know, as they won't swallow the food any longer. It's the best you can do to support him while you sort things out. I'm a bit concerned that the vet may not have enough guinea pig experience to really help him much. Is there an exotics vet in your area who has specific experience with teeth? No disrespect to the vet, but doing a partial dental because you don't have the tools to do it properly and then essentially writing him off doesn't sound, to me, like someone who has a lot of experience with this particular problem.
There are a lot of reasons why dental issues can suddenly materialize. I've had dental issues with two of my pigs. The first developed a large abscess right at the beginning of her dental issues, so the abscess was plainly the underlying problem. Unfortunately, she didn't do well and passed away about 10 days following surgery to lance and drain the abscess... I didn't know as much about hand-feeding and abscess care as I do now and the vet didn't know as much as he should have known, and I don't think that helped her out any. Sundae presented differently, more like your little guy. She just went off her food, was losing weight, and was having difficulty chewing (she would pull her lips back as if food was getting stuck and would paw at her mouth.) I had since switched to an exotics vet, who was able to ascertain that she had overgrown molars on one side and a molar spur that was cutting her her tongue and was painful for her. That did the trick for about eight weeks, and then we were back with the same problem again. About eight weeks after that, she developed a lump on her jawline and was treated for an abscess. The vet and I are both pretty sure that the abscess was brewing down in the deep tissue for months and was causing her dental issues (it hurt, causing her not to chew on that side, causing her teeth to overgrow on that side) but we had no real way of assessing that until it became visible. We did have to syringe feed her for awhile, and we had to squeeze out that infected abscess pus for a long while for it to heal, but it's now been over a year and she is fine, no more dental issues since the abscess lancing. I'm not saying this is the specific reason for Diesel's issues, just that tooth issues can spring up and there can be underlying reasons that aren't readily apparent. Really the best you can do now is to keep on syringe-feeding him to keep his guy moving, keep offering food, and maybe see if you can find a vet with more experience with guinea pig teeth.