Originally Posted by TxHorseMom
This may sound weird, but having the bit too loose in his mouth may cause problems too. There are suppose to be 2 loose wrinkles in the mouth when a bit is properly fitted. If it is too loose and then you pick up the reins, it can "clank" in his mouth against his teeth. I have to agree with your vet to a degree. At this age he probably doesn't need floating, but it could be wolf teeth as suggested before. Good luck.
Yep. I was going to say this - but the loose wrinkle or two is an individual thing depending on the horses mouth. The first time you bit a new horse, you sit the bridle and bit where you think it should go, then open the lips and LOOK.
I am betting that either the bit is handing too low and clunking his teeth, or the bridle is too loose and the whole shebang shifts a bit and clunks his teeth.
Again, general rule of thumb for an Englis bridle is two fingers ( flat) under the nose band, a fist under the cheekpiece.
Your best bet is probably to have the bridle on as you normally do when the vet arrives.