My working er spaniel eats poo!?

My lovely working er spaniel (7 months) eats his own poo. He is a clever and lovely little chap who, when we are out walking or I am with him is rarely even interested in any kind of poo. He understands the leave it command when I am by his side. However, if I leave him to go in the garden unguarded he will always find some poo to eat - whether it's fresh or been there for a while. This has been difficult considering the recent weather. I have been trying to call him back in with the lure of 'biscuits', which works, but usually after he has chosen a juicy morsel to munch on the way back in. I'm nervous of making this a game or making such an issue of it that he starts to chew on purpose. I have changed his food, which worked for a short while. Any suggestions to try would be gratefully received.