Thank you all for your advice. He doesn't eat the dung in the stable, only when turned out and I turn out at 7.00am and they come in at 8.00pm(he has a locking stifle as well!! and I turn out come rain or shine). I feed hay in the field and they have the paddock blocks as well. I had thought of using a probiotic and have just recently added this to his diet and hopefully it will help. He does appear to be well and healthy and the only reason I know he is dung eating is that I can watch the horses from my lounge window!!, otherwise I don't think you would really notice as it is not something he does constantly, I certainly haven't noticed a reduction in my work load as a result (mores the pity). I know I should have him tested, but as he looks well I was trying to sort it out with out the added expense. It's interesting about the teeth though, as I had them done shortly after he came to me and my horse dentist said they were bad even though they had been done 8 months previously by the same dentist, perhaps this is something to do with it. He has been kept on a comprehensive worming program all of his life, and I have continued this, my main concern was that he is eating the dung and therefore increasing the chances of infestation.
Thanks again, I'll let you know how we progress.