please could anyone advice me on kennel cough as i have a9 month old border terrier who has this we have been to vets and we have antibiotics but he still sounds terrible

Did your vet recommend Benelyn cough mixture?

If you do a search on this forum you will find plenty of discussion and recommendation on this.  I gave mine Benelyn and got a satisfactory result.  I can't remember whether I gave him dry or chesty cough tho!