DS confused about which end it comes out from

[tigger15]

DS really seemed to have got the hang of toilet training (at over 3 1/2!) but this week has regressed on this one point. He insists on doing wees standing up like daddy but for the last 3 days in a row has done a poo instead - all over the floor. I have tried to explain that big boys can do everything sitting down and dh has repeated it but it is not going in. He also had a d&v bug at the beginning of the week so I am not sure whether this episode is related to a still dodgy tummy.

Does anyone have any tips on how to resolve this one remaining issue? He is mainly dry at night so I really thought we were there.