It isn't diarrhoea, as such, in that he isn't pooing all day long (twice a day, same time of day as normal), but when he does poo it is really liquid stuff. He had a tiny drink of cow's milk on Thursday, and I put it down to that, but 3 days later things are still the same. I can't think of anything different that he's eaten. Possibly more tomato sauce-based meals than normal.
He's having a tomato free day today to see if that helps, but I don't really know what else to try eliminating.
Are there foods that can make poo runny and should be avoided? What could he eat that might firm things up a bit? I am very inexperienced in poo matters.