Hi,
I do not want to worry you,but if the suture line is looking red it could be infected.The "wet" head could be because he has a high temp.
Is there another vet in your area that knows more about piggies?.Do you have a gp rescue near you?,you could ring them and ask them what vet they use.
I realy think this little chap needs to see a vet/rodentologist who knows what they are doing.
Push the fluids ,especially the rehydration solution,get as much into him as you can,and also get as much syringe food into him as you can.I know it sounds unkind to keep pushing a syringe into the mouth when they are not well,but it is imperative to keep the fluids,and food going into him.Piggies need to be stimulated when they are unwell anyway,otherwise they will just give up and sit in a corner.Stroke him and talk to him while you are feeding.Talk any rubbish in a soft low monotnous voice,I am convinced thata they find this comforting.
Re Arnica, I have not used it for a long while,and the only thing I have used it on for a piggy is bruising on a limb.I do not think it is suitable for sores etc.
Hugs from The Epsom Mob