Jambuttie, I am very surprised that your dentist & health visitor are fine with you giving listerine to your 3 yr old. I have been a dental nurse for 15 years & we have never recommended something so strong for a child so yound. Regardless of how much he uses.
You said in a previous post "Also as he sleep with his mouth open plaque becomes hardened onto teeth even though we brush them it wont remove it"
Does he have harded build ups on his teeth now? If so you are not removing all the plaque when you brush his teeth. Plaque is a soft off-white substance that clings to the teeth. Mostly everybody gets plaque, which can be removed by brushing. If it is not brushed off it will harden, this is then called calculus, more commonly known as tar tar.
It shouldn't be building up on a childs teeth, regardless of wether they sleep open mopunthed or not. I sleep open mouthed, as does my daughter & we do not get hardened build up, simply because all the palque is removed.
Once you have brushed his teeth before bed, does he still have milk after? If so you really need to stop this as he will be sleeping with a mouthful of plaque & will get cavities.
Having read your posts I would advise you change dentist IMO!