Kiwi-fruit is a mild natural laxative but you'd prolly have to give him a few of them )

The highest doses of fibre are found in fruit/veg so maybe up his intake of these for a couple of days - like more than normal.

From memory I got told with the bananas, that it's only the harder more unripened (sp?) ones that don't help with constipation.....so I think really soft bananas are ok to eat.