White sore under dogs tongue

Just noticed today. No changes in behavior or appetite. Can someone identify what this may be? He is 12yrs old with no health issues besides having majority of his teeth removed 2 yrs ago due to periodontal disease (neglect previous owner). Does occasionally have (rare) acute pancreatitis. 

Hello!  I'm not sure what's caused that but a vet-check is a good idea.  It could be inflammatory (eg infection, burn or from the sort tissue pushing against a tartarous tooth or a foreign body) or neoplastic (cancerous or benign).  Sometimes, worryingly, mouth ulcers can happen secondarily to other illnesses in the bloodstream eg due to a high urea in kidney disease.   Therefore a vet check is a good idea.  Please do ask them to triage how urgent they feel it is.  Wishing you all the best.