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Welcome to Just Answer! I would like to help you and your dog with this problem.
What you are describing is probably an oral tumour. As with tumours elsewhere in the body, some are benign and some are malignant. One of the most common ones is something called Epulis and this is genereally benign.
This is something that should be seen by your veterinarian because removal may be necessary. If it is benign, then it will not hurt him if it does not get removed. If it is malignant, however, it could spread elsewhere in his body and be life-threatening.
Because this could be serious, I would have him examined soon. I hope that is is benign epulis, but it will need to be tested in order to know this.
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Best wishes! Fiona