Went to the vet today, and he said it is an epulis - a common oral tumor that is usually non-cancerous. However, he has to have it surgically removed and biopsied; if it has grown into the bone, they may need to remove some teeth and jawbone
I am praying it's tissue only and doesn't affect his front teeth. I feel absolutely awful, wondering how long this has been there and if I should have caught it sooner. Anyhow, he is scheduled for surgery on September 25. I will update.