Meggy my ds has had this for about 3 months now, oddly enough from when he was around 11-12 months too!
The thing which appears to make a bit of a difference is the amount of fruit he eats. He was having a3-4 peices of fruit a day, so I've tried to cut down a bit. Also cut out fruit juice.
It does seem to make it all a bit more solid. But it still smells dreadful tho!
Today I had a lovely one that was all down his legs and inside his trousers. They were an old pair so I'm afraid I just threw them away it was that bad!!!