If a stone has definitely been ruled out for Cashew, then indeed Interstitial Cystitis is a bone fide train of thought. I note he is on a hefty dose of Metacam, but is not responding, this is very common I am afraid with this type of cystitis, as the bladder becomes so thickened with inflammation, in some cases adhesions/blisters will occur and stick the bladder to the bowel wall, thus he may not necessarily be crying when urinating but also when defecating.
I have had a lot of patients this year with this type of problem, all the usual first base cases looked, urinalysis, x-ray, and bladder flushes, and even then some of the patients continue to present with pain.
What to do, well first of all and I say again, provided stones of bladder and kidney have been ruled out, this little pig could do very well on Cystease tablets, yes they are a cat medication, but we have had tremendous results with them as they coat the bladder giving protective covering. They must be cat Cystease S, 125 mg. Dose 1 capsule direct into mouth, or watered down and syringe fed daily, usually this will be for life. Combine this with preferably Rimadyl pain relief, 10 mg 2 x day, Rimadyl stays in the system longer from our experience. 40 mls of water either mixed with cranberry juice or just water daily, add into the fluid treatment any Cystitis preparation for humans, I use Cystemme, I give this daily for 2 weeks, yes longer than the norm, but the idea is flush, flush, flush that bladder. 1 sachet over 2 days mixed up as directed will give an idea as to what intake I am advising to aim for.
We have also recently been looking very carefully at diet, in severe cases, we have removed all dry mixes, including pellets, strictly no fruits (sugar and acids irritate bladder problems) and absolutely no vegetable that contains sugars, carrots, bell peppers to give you two examples. And it does seem to work. Leafy green diet,, readigrass, decent hay, are more than satisfactory to keep the teeth in check.
It is very hard work as you can imagine, but from the pigs I have treated I would say our success rate is running at about 85%, for Cashew perhaps worth a try.
This is the product to which I refer.
http://www.vetmedsdirect.co.uk/products/Feline-Cystease-S-Capsules--125mg-x-100-capsules.html