If you're confident with the vets diagnosis then there's no reason not to syringe-feed - I had to ask and mention the concerns about syringe-feeding bloated piggies to better understand his condition, as the advice you receive depended on your response. I hope that makes sense!
Acute cases of bloat tend to require fairly intensive care, regular medication and syringe-feeding, but it can be overcome. Do you know the name of the "laxative" given? One of the most common medications that appear to be given in such an illness is Metaclopromide, which helps stimulate the digestive system in an attempt to get it moving again.
The most useful medication I've personally used to treat bloated guinea pigs is Buscopan. It's usually marketed for IBS symptoms, but one tablet crushed (and syringed in 1ml water) helps significantly with my two guineas who suffer chronic bloat. However I am unsure if Buscopan can be given 'alongside' Metaclopromide or other medications, so perhaps it is worth discussing with your vet.
When you syringe-feed, alternate mouthfuls/syringes of food with a bit of water. Also syringe a little water into his mouth at the very end of the syringe-feed, to clean the back of his mouth a little. (It seems many piggies are terrible about letting food sit in the back of their mouth for a long time!) How much syringe-food are you able to get into him at each feed, and how often? Piggies who are not eating anything at all for themselves generally require at least 60ml per day, spread out between at least 4-6 feedings.
The change in diet is a possible cause, some particular foods or changes in the diet can upset the digestion of some guineas. I know a handful of people who feed rabbit food to their guinea pigs but it isn't something I would usually recommend, even with the Vitamin C supplementation.
I hope he's able to pick up very soon.
