Just to let you know one of my guinea pigs Skye recently suffered from dental problems. She lost a lot of weight (was about 630g approx at her lowest) and when I took her to my local vet they trimmed her teeth too short and then told me the best thing to do would be to have her PTS.
I was contacted by one of our members Debbie (furryfriends) and she helped me arrange an appointment with her fabulous and very guinea pig savvy vet in Northampton. Skye and I went on the train and Simon was excellent. He filed down Skye's slightly overgrown molars and monitored the regrowth on her teeth that had been trimmed a bit aggressively and within a month she was almost back to a normal weight and was able to eat and chew again. At the moment she's very well in herself, she's living with the other piggies again and is now pretty bossy at feeding times, she has no problems eating at all and is back to being a bit chunky again. There is hope and there are vets out there that will be able to help and offer appropriate treatment. I'd say if he wants to eat the syringe food and seems okay in himself then that's a great start.
It would help if we knew where in the country you are located so we can give you some better advice.