Are you sure it is real diarrhoea? When ds1 was in nappies, his poo was alwaays big formless splodges in the nappy, and I thought he had constant diarrhoea, but in actual fact, by the time he was doing them into the loo, I realised they were just loosely formed and had been sat on, and in the loo, they managed to hang together OK. Does that make sense? he did have trouble getting used to doing them in the loo so I had frequent accidents in pants for some weeks, but it wasn't impossible to train him.