Mouth sore

Hi I was wondering if anyone know what this could be and if I should take him to the vet? Thanks in advance 

Hello!  Unfortunately this picture is blurred in the vital place, but I assume that the topmost layer of lip-skin, the 'epithelium' has come away, leaving it raw: 'an erosion'.  If this is the case, there are many possible contributory reasons including a burn, sunburn, allergy, the effect of skin rubbing against the skin opposite it (usually connected to the shape of the mouth), a nail-scrape (eg done when scratching the mouth), and of course secondary infection such as bacteria or yeast.  Cancer is also a rare possibility.  Such lesions can often become exacerbated, especially if scratching and infection are involved and I would absolutely advocate a vet trip in order to get some appropriate treatment or tests.

Sometimes, particularly in the event of a sting or bite in the area, it is also possible to get an eosinophilic reaction.  In general, lesions may respond symptomatic treatment; if, for example, this fails or the lesion proves recurrent, or if they have reason to be particularly worried, vets may take a biopsy to find out more.

Hi. Did you ever find out exactly what the mark was? We are having similar problems with our dog and just can't seem to get any help from vets here in our local community in Italy. Thank you