Why can't he close his mouth?

[jAfFa CaKe]

Hi all,
My axolotl, about 3 years old, has recently not been able to close his mouth. He can close it mostly but his mouth is still a little bit open, he also seems to open his mouth, as if he was trying to eat about every15/20 seconds. He ate a worm today, I tried to feed to feed him another and he did try and eat it but spat it out and has not eaten another.
Parameters:
Ph: 7.6
Ammonia: 0
nitrite: 0
Nitrate: 10-20
Any help is much appreciated.
PS: I have another axolotl in the same tank, it is fine and eating as usual.
I will get some pics soon.

 

[rachel1]

How long has he been having trouble closing his mouth? Could be something like an abscess or a tumor, or there could be a foreign object in there keeping his mouth open. Can you see inside his mouth to determine if there's anything in there?