I would trust an ENT more than a dentist, but its totally up to you. If you feel that much anxiety about it, keep trying to get a biopsy so you can put your mind at ease.
Can you see the patch in question with a handheld mirror? Personally, I'd get a friend to take a picture, and look again in two weeks and compare the photo with your mouth to see if it's healing. If it is, you're probably OK to wait the three months to visit the ENT again and have them tell you the same thing. If two weeks go by it isn't healing, and/or looks like it's grown, I'd go back to the ENT and insist on that biopsy.