I am sorry you are in this position.
Definitely seems like you need to get him back in and seen by a vet for his legs. Your best bet will likely be to get an x-ray and then you and your vet will be able to go from there.
The good news is that guinea pigs can actually continue to move around without/reduced back leg use as they bear most of their weight on their front legs. My grandmother had a piggy who was almost completely paralysed in his back legs and he lived a long and happy life, although he was on a large amount of painkillers his whole life.
I have had a piggy with quite bad ongoing gut stasis for around a month. He was on 0.12ml of Dog Loxicom twice a day, 1ml of Emeprid twice a day and 0.1ml of Cisapride three times a day by the end of the month.
The more critical care you can get into your boy the better, to keep the gut moving as best as you can. When my boy was not hardly eating I was feeding him around 60ml of critical care a day. Even when he was eating much better on his own he was getting around 30-40ml a day to help stabilise his weight.
Gut stasis is commonly known to be an issue caused by another ailment in the body, but it can come out of the blue.
In my case we went around a month of medicating before looking into investigative procedures because we were having no success with it, but hopefully the medication will work for your piggy and have him feeling better soon.
As for his painkiller, I would recommend bringing up an increased dosage to twice daily with them, but they will be best informed about this.
