It doesn't look like mites or a skin condition - there's little hairloss around the wound, the skin isn't red or inflamed, and the edges are clean. It looks more like a cut or a bite than a mite problem. They look quite nasty and if he were my pig I'd definitely be taking him to my out of hours vet before Tuesday, mainly due to the location of them. On his underside, it's going to be difficult to keep them clean and uncontaminated, especially if he's anything like my pigs who like to lie in their urine etc.! Also size wise, they are full thickness wounds of a pretty decent size, on us, it'd be like a 4-5 inch cut on your abdomen.
In terms of what's caused it, it's an unusual place to be bitten (does he have a friend?). If he's been in the run, if it's a wooden one with a wire mesh, I'd be checking for any exposed sharp pointy bits. If he's got a friend, I guess he could have gotten a little "over friendly" at the wrong end to have been bitten there.
With regards to an emergency service vet, if you call your usual vet, there will either be someone there to answer or there will be a recorded message with instructions (have a pen and paper ready as there may be another phone number to call!)
In the meantime, keep him as clean as possible (just some cotton wool and cooled warm salt water - 1tsp in a mug of water), and bed him on something that won't contaminate the wound and that can be changed daily - fleece or newspaper would be good.
I do think mites as touching his back makes him turn round and try to bite me a bit at the moment cannot get to vets bcos of bank holiday so I guess even spot on atm is better than nothing
