tic or allergy?

My son is 8 and he's had this cough that sounds like a sneeze for a long time now. It started a year ago when he got braces. We took him to the allergist again (he had already been diagnosed with asthma and seasonal allergies) when he was 3. They discovered he had a new very high latex allergy. The braces came off and the cough stopped. they assumed it was because of the elastics in the braces.  Then 5 months later, he developed a cold and now the cough is back. It sounds so much like a sneeze people often say god bless you. He hunches over with a force when he coughs and says he can't stop it and that it feels like a tickle and a whistle at the base of his throat. We went to the Dr. they gave us zythromycin for a sinus infection, claritin and prendisone. We finished up everything, did the warm mist humidifer and it's still going strong. I just ordered "tic tamer" b/c one doctor over the summer said it was a tic from getting braces and an expandor on the roof of his mouth, very uncomfortable made him develop this "tic". He does not cough at all at night while he sleeps but the minute he wakes up it starts and goes all day. It's a very annoying, loud, jerking forward type of cough that makes your throat hurt when you hear it. We have tried inhalers but it's not in his lungs. We took him to the ENT who said he had acid reflux so now he takes prevacid before breakfast. I can hear the whistle in the base of his throat when he breathes in so now I'm not so convinced that it's a "tic" cough. I'm at my wits end! Taking him back to the doctor today to see what they say. The braces are supposed to go back on in a few weeks so now I'm not sure what to do. He's not having any issues at school or at home, he swears he can't stop it and it's not voluntary. When I ask him to stop it right before he's about to cough, he can't stop it. Stay tuned....