I'm pretty sure that both of the boys have toddler diarrhea, which is fairly common and not serious, although it does make for a lot of unpleasant nappy changes.
Looking into it on the internet I came across a woman on netmums who had a daughter with a similar thing. After much 'research', she 'discovered' that her daughter was fructose intolerant.
Now the daughter has a diet which contains absolutely no fruit or vegetables (including juices), almost no dairy and no brown bread (or anything else that could be considered high in fibre).
Apparently this has 'solved' the diarrhea problem. Can't help feeling that a diet consisting pretty much entirely of meat, rice and pasta will be storing up a whole set of other problems for the future!
Definitely think that Doctor's advice is a must before putting a toddler on such a restrictive diet :/