Thank you for your kind words, Sawdust 
Hello again everyone. I thought that a quick update on Pixie would be nice, especially because so many people have helped so much in the first days after his second dental work that were critical and also because other people could be just as worried and lost as we were then.
It is now 10 days after his molars were filed down for the second time with general anaesthesia and Pixie is eating all the kinds of foods that he used to and almost the normal amount.
After the intervention the doctors advised giving him 21 ml of Critical Care food a day, in 3 doses but he wasn't doing very well, and doctors said that there was nothing else to do.
On a different guinea forum I was strongly recommended by people who have had guinea pigs for many years to feed him more CC, because the only way for him to start eating was by feeling strong again and fully functioning.
It was a very hard decision to take to go against the doctors' advice but 48 hours after we started feeding him 120 ml Critical Care a day, 20 syringes every 4 hours + 5 syringes of 7 ml water, he started to eat a bit of fresh grass, then the next day a bit more and he dared to eat something else too, and so on, he progressed every day a bit more.
We gave him and still do constant attention, he spends most of the time during the day in my mother's arms, in his little sponge house covered with soft blankets, constantly being offered new fresh food to stimulate him. My mother is retired and this helps too.
He wasn't very keen on the Critical Care food but he was always fed the right amount. He sometimes ran after being given one syringe of CC in his house on the sofa and we started to offer him normal food to eat on his own in between the CC doses and he accepted it and started chewing. His feeding times took much longer than usual but he got more and more used to eating his normal food on his own again. We've also gradually decreased his CC doses.
We are very happy now, he is doing great, today is the first day that he hasn't received any Critical Care food because he's eating on his own, his grass, hay, dry food, carrots, parsley, etc.
Thank you again for your great help in those moments when we were desperate. All the best.