A few black teeth is normal for a beardie. If there is no redness + pus and he's eating, there's no mouth rot. You can buy a product called maxi guard and gently try to brush is teeth with a very soft toothbrush for children [ with or without maxi guard, but never with toothpaste ]
A bit of saliva can be present at times, but has he had labored breathing ? Any wheezing or clicking noises and opening his mouth every few minutes ?
I really don't think there's any problem if you don't see those symptoms. BTW, be SURE that your ver is a true, qualified reptile vet or they can usually do more harm than good while you pay them big money.