First thought is teeth. He's the right age for his teeth to start playing up
from time to time, and normally if there's no signs of any other illness/
infection chances are it is indeed teeth.
I've had a nightmare trying to get homes and vets to comprehend that - I've
found an incredibly good piggie (and degu!) dental vet, though he does still
use gas to do a pigs teeth but better a skilled vet using gas than an idiot
vet doing it without in my opinion.
Without going into a rant, one of my lot stopped eating. She was clearly
hungry, but would not eat for herself, and waggled her head when syringe
fed. I suspected a tooth problem, but local vet after local vet said her
teeth were perfect. One even did a full dental exam and burring! Long story
short, eventually I told the local vets exactly what I thought of them, and
through the local rodentologist found a vet with a good reputation
(unfortunately a 40 minute or so drive away). He immediately spotted an
infected ulcer in her mouth (that another vet had told me was bruising from
me syringe feeding!) and was knowledgeable to suspect it was caused by a
tooth spur. The next morning she went in for the op, and within an hour or
so of coming round was head in the food bowl, for the first time in at least
a month... It turns out she had a tiny, but destructive spur on one of her
molars.
I'm currently trying to get family to acknowledge they need to get one of
their boy's teeth done. He appears to have 2 growths (I won't say for sure
tumours, but that is the most likely at present). He stopped eating a few
weeks ago, at the same time as a lump appeared. That lump then went away
leaving a scab (sebacious cyst anyone? - Another missed diagnosis by a vet),
though another, internal lump appeared around the same time. Ever since,
even though he's picked up in himself, he's been unable to eat properly and
has had to be syringe fed. I'm in contact through email, and I knew he had a
tooth problem from what was described. He came down when they visited
recently, and I said he had a tooth problem. I could tell from how he was
reacting to food in general, and especially when syringe fed. Their usual
vet had said his teeth were fine. I persuaded them to go to "my" vet, who
found (not suspected, but found, ie saw) 2 or 3 small spurs. Annoyingly they
had to drive home the day he would have had his dental, so he was placed
back in the hands of frankly, halfwit vets. First vet said he had spurs
(same vet as said he didn't previously), but that he would eat on one side
to compensate so not to worry. Second vet refused to do the dental, as "He's
not in a good enough condition to make it through the operation". FFS, if
the pig was perfectly healthy he wouldn't have a prblem that required a
dental!!! Same vet also said again that although spurs were present, they
wouldn't be the reason he's not eating properly.
Now, he will eat from a syringe, but is in some discomfort when chewing. He
will pick at his food bowl, but only the easiest to chew and swallow foods.
He will eat various fresh, but oly that which is most flavoursome or easy to
chew. It seems that the various vets he's being treated by at present are
putting the lack of eating down to a tumour (still not 100% diagnosed as
such, and not attached to his stomach or otherwise impinging on anything
that would suppress his appetite, beside being potentially operable) rather
than having any clue about just how a pigs mouth works (or how a pig works
at all...) and understanding that actually the tiniest, barely perceptible
of spikes/ spurs can lead to major eating problems.
Why have I typed all of that? Well aside from wanting to vent, it's to make
clear that your boy may well have a very small, simple to rectify, but
significant in terms of eating tooth problem. And if no other reason can be
found for his loss of appetite, keep pushing till you find a vet who is
truly skilled in cavy dental work. As you're UK based (I assume, as you're
using ntl), contact the CCT (http://www.rodentology.org.uk ) as they may be able to
recommend a vet or even have a member local to you who could see him and
check/ do his teeth (they are trained to do dentals without anaesthetic,
which is a very good thing).
If he really isn't eating and starts to lose weight (as you suspect a
problem weigh him daily - More than 20g or so loss is a worry IMO) you might
have to start syringe feeding him until he can be diagnosed/ cured. It's not
a fun thing, but once you've got the hang of it it's quite straightforward.
Good luck