My 2 year old is seeing a speech therapist. He often has his toungue resting on his teeth, especially when he's consentrating. I didn't think anything of it because his cousin and his uncle (both on my husband's side) do the same thing. The the therapist mentioned it, so I asked. She said in my son's case, he has low muscle tone in his chin & jaw area. Because those muscles are weak, he doesn't have the strength to articulate properly. It will just take time and practice to strenthen those muscles. I asked my husband about our nephew and his brother, and he said they both were speech delayed.
No idea whether we're talking about the same thing, but it couldn't hurt to ask the speech therapist to explain it better. Talk to him/her about your concerns and what the ENT has said. Often times, medical professionals don't give you details unless you ask, so he/she might not even realize you are concerned.