Hi! I am very sorry for the ordeal he has been through. Urethral bladder stones for boars can be life threatening because of the length of a boar's urethra and the fact that they have got an inglenook, so he been likely through a major operation.
He is obviously still in major pain, which is causing loss of appetite. It is important that you hand feed him to keep the guts and his strength going. You can give him extra vitamin C, too. I assume that he is also on an antibiotic, for which we recommend to give a pinch of probiotic about 1-2 hours later in order to bolster the effects.
Post-op guinea pigs often like to stabilise their guts with very rough and nutritionally poor fibre, so a toilet roll inners or some other brown cardboard would be ideal for him to gnaw on. Make sure that you weigh him daily at the same time in the feeding cycle to keep an eye on his actual food intake.
See your vet as an emergency if there is a sudden change for the worse or if there is no improvement over the next few days.
Here is our complete hand feeding guide:
http://www.theguineapigforum.co.uk/threads/complete-hand-feeding-guide.115359/
You may also want to think about a low calcium bladder stone diet. Some people prefer the the ph : ca diet, but it has never worked for me. You can find tips for either diet in our Caring for an Ill Piggy section. Filtering the water can help quite a lot towards preventing the build-up of a stone, too, as well as limiting any alfalfa-based pellets that are quite high in calcium once he is eating normally by himself or he is on proper recovery food.
We also recommend to give vegetarian glucosamine as a food supplement to coat the walls of the urinary tract.
Wishing your poorly boy all the best and a good recovery.