Cookie has started to eat her own poo too recently, I spoke to my dad who said he thought it was due to a lack of vitamins or minerals in their diet, so they eat their own mess to compensate. Im not sure how true it is but I have heard it a few times. I stopped cookie doing this by changing her food, she was on dry food only before but now i give her some tinned meat, or add gravy to her dry food. this seems to have helped for now.
There was another thread on this subject recently - it may help http://www.labradorforums.co.uk/ftopict-847.html
It's quite normal for a dog, it's only we humans that think its disgusting. If he's eating his own, I'd make sure you pick up after him in the garden. Some say adding pineapple to their diet helps as this is high in digestive enzymes - one of the theories it that faeces contains alot of undigested food, so for a dog it is a food source.