I have a boar with a similar problem at the moment so you have my sympathies.
Because of all the problems with his winky, it sounds like he has now got a urine infection in his bladder which is painful, smelly and can cause blood in the urine. Your vet has given him metacam for the pain and tribrissen (septrin) anitbiotic for the infection which is good.
However the grinding teeth (particularly when he is trying to wee or poo?) indicates he may still be in a bit of pain , particularly when he wees, and until the antibiotics get to work and dampen down the infection he may have a couple of rough days ahead of him. How much metacam are you giving him in either mls or drops and how much does he weigh? (You state in your earlier post it is 0.5mg/ml so I assume this is the cat metacam?). Depending upon you answer it might be worth ringing your vet and asking for permission to up the dose for a couple of days and this I think is the most important courseof action as reducing the pain will make him feel a lot better.
The pain is stopping him eating and he is losing weight. So the blood in the urine can cause teeth problems rather than the other way round. You do need to syringe feed him to keep his guts going untill he starts to eat again by himself...try and get 120ml of lquid feed down him per day, feeding about 20ml in one sitting (as that is about the average capacity of their stomach)
If there is no improvement after a few days, the vet may need to do a urine analysis and culture to find out if there are any bugs/crystals and maybe try a different antibiotic. If no bugs are found, he will need an Xray to look for stones (edit - see you;ve already had ultrasound and it was clear - however urine infections can cause stone formation so don;t rule this out)
Finally, home products that could help include unsweetened cranberry juice which stops the harmful bacteria from gaining a hold on the bladder lining). Some people use the Ocean spray unsweetened one, I buy cranberry concentrate from Holland and Barret and either mix some into the liquid feed or into the second product "pearl barley gloop" below.
Buy dried pearl barley (usually next to the lentils) and boil it up for approx 45 mins until it is soft and mushy (you;ll need to keep topping up the saucepan with water). When it is soft, make sure you have some remaining water still in the pan and then drain the pearl barley through a sieve, mashing it as you go and catching the sticky watery gloop that comes off it in a bowl as this is what you want. Store in the fridge for no more than 3 days and syringe feed 1.0ml a couple of times a day to your piggie. The pearl barley gloop allegedly contains compounds that help coat the bladder lining and soothe inflammation. I have found this really did help a couple of my pigs with badder stones/urinary infections.
Longer term: When you say he had an op, I'm assuming they knocked him out/sedated him and just thoroughy cleaned and inspected his bits....rather than any actual surgery? If that is the case, then when he has stabilised/got rid of the infection, you need to keep an eye on his winky and if he is one of those pigs who is prone to stinky willy problems then in future cleaning with a cotton wool ball soaked in dilute 2% hibiscrub (2ml in 100ml), drying gently followed by a thin application of sudocream (thanks suzygpr for that tip) can help prevent it ulcerating again.You should only need to do this once or twice a week and he may become able to keep himself clean after a bit.
HTH
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