A few points to consider... sorry for the formatting - I'm just trying to keep my thoughts in order!
Enough syringe food to maintain weight
I had a little sow that wasn't eating and her teeth overgrew as a result. I'll say at this point we think her problems were possibly not dental in origin but maybe some issue with swallowing. Even with a tiny amount of sloppy syringe food she just seemed to chew at it for an age. However the overgrown teeth happened as a result. She survived exclusively on syringe food for about 6 weeks but needed about 45ml each day to maintain her weight... this was the important thing, she wasn't losing any weight although she maintained at 950g by this point rather than the 1.2 kg she'd been in her prime. That was the max volume she could take and we started 6-7 in the am and finished the last feed by 11pm so we didn't go through the night. I added in probiotic (she was on ABs too) and vitC to the mix. Your boys seems very light for 2 years old... could he take more syringe food? Will he eat it from a bowl in the night or will his friend pinch it?! As far as I can tell after 6 or 7 ml they start to get a bit full - Ivy took so long to feed because of the chewing thing that it took about 1 1/2 hours to get 10ml down her. Her healthy cage mate gobbled down any spilt drips though!
*Just seen your last post - he's getting plenty so he's a hungry boy!
Painkiller
If the loxicom is 1.5 mg/ml it's a decent dose but the vet could go higher with no ill effects... but that might not make the difference. If it's the 0.5mg/ml it's pretty low.
Front teeth can obstruct chewing
My girl had her back teeth done as soon as she stopped eating (we obvs thought it was just teeth at first) but not the front ones as they didn't need it. However on a check up a few weeks later a different vet observed that, "this guinea-pig can't close it's mouth!" because the front incisors had grown so long. Of course, with no biting or chewing this is what had happened. I had noticed Ivy trying to bite at wooden things for a few days but didn't put 2 and 2 together. They burred them short (too short actually) w/o general as you just need someone to hold your angry piggy while the vet does the work in a few minutes! If they are too short it's not a disaster as they grow back extremely quickly but if they are too long they can get in the way and piggy can't pick up food. Some pigs on here have had incisors removed completely and adapt very well. Did your vet cut the incisors down too?
Spurs on back teeth can cause ulcers in the mouth
Sometimes pigs develop little (or big!) spurs which stick out to the sides and dig into the soft flesh. The back teeth overgrow and need to be ground back but the spur also needs to be removed or piggy wakes up with his problem still there. Did your vet mention anything like this?
Sloping incisors can point to the problem
Sometimes piggy's front teeth seem to slope to one side... this is a result of pain or problems on one side of the mouth and piggy tries to chew on the other side to compensate. Did you see anything like this before he was done?
Dental abscesses a hidden problem
These come up time and again on the forum for persistent tooth issues. I haven't had this experience myself - as far as I know! When Ivy first stopped eating the vet felt carefully round the jaw for swelling, tenderness, points of pain etc that might indicate an abscess but found nothing. She was prescribed antibiotics anyway but for her they made no effect. However, dental root abscesses can be very resistant to treatment and some ABs are better than others. Did your vet check for these and have you had any antibiotics at any point anyway? Ivy got hers on the off chance there was some hidden infection we couldn't spot.
Teeth are a tricky thing to get right
It's what no-one wants to hear but it's very true... there are people who have posted on here and ended up going to a different vet (possibly a specialist they had to travel to get to) to finally identify the problem.
'Feel' how he is chewing
When you give him some syringe food rest your fingers lightly behind his ears on the bald patches. You should feel the vibrations as he chews and it should feel smooth, even and very fast. When Ivy's incisors were too long her chewing action was slow and interrupted as the front teeth got in the way. The 'grind' felt rough and intermittent. After she had them burred she came home (absolutely furious) and chewed fast, even, and smooth - I was so happy when I felt the difference - I felt sure tomorrow would be the turning point but unfortunately it didn't help her to swallow and the feeding times didn't speed up.
Here is a little gif of my other piggy Zara. She had a cracked bottom incisor that hurt her and she stopped eating - there was actually nothing wrong at the back. This is from when I was syringing her before her vet appt and you can see how she pushes her jaw forward so the painful bottom tooth is kept out of the way. Her chewing is irregular and she chews with her mouth open... have you seen anything like this?
