One of my puppy buyers called last night asking about a problem with their dog a DDB he is now 21 months , I was aware he had a problem with his mouth a few weeks ago as they called to say he had been to the vet . They thought that their Llasa apso had been licking his mouth too much and had made the inside sore , the vet examined him and couldn't find anything out of sorts but gave a course of antibiotics. However now they say you can touch the outside of his mouth fine but if you try to touch the inside to have a look it is hurting him , and he can only open his mouth approx 1 inch but he is managing to eat fine. 
I haven't had this problem before but was thinking he may have a problem with a tooth , my husband said lockjaw but I just don't know. If it were me I'd take him back to vets for further examination , but they are asking me as I am his breeder and usually can give them advice / solution but this time I'm stumped .
Just thought I'd post here to see if anyone had heard of this happening before.

It could be a sore tooth, possibly an abscess, or there's also the possibility of Masticatory myositis.

PM me if you want. I have experience of MMM
Diane

Thanks have pm'd you

Thanks jeangenie the link is very helpful , I'm hoping it is just a tooth problem.

Spoke with the owner last night and after discussing everything they are taking him to see the vet that I use on Thursday.
I will update when I know more.

Well owners took him to the vets yesterday and said it possibly could be MM ,but also mentioned lion jaw and it is a problem often seen in westie's , he is on the steriods that have been recommended and is being reviewed a week on Saturday by the same vet , she is then going to decide where to go from there with probable xrays.
I do hope he gets some relief he's such a gorgeous boy :)