The vet didn't think there was an abscess and couldn't see any inflammation in his mouth but we have some metacam so that's a good start (and he was very good to dose!)
The weight loss started before his teeth were clipped, I noticed on Sunday that he hadn't eaten and he was at the vets Monday where they saw his top teeth had grown curved around towards the roof of his mouth so that was the reason why he hadn't been eating properly and losing the weight (it was a slow decline until really recently, corresponding with the teeth getting longer and more curved)
Since Monday he's not really been right though which was why he was back today, hoping the painkillers will help. The new diet will be arriving tomorrow so he can have that plus some baby food