It is actually normal for a child to do this.  Back in the caveman days (sometimes wonder if we ever left them! lol), we would have pooed standing up, or squatting.  As time has gone on and we invented the toilet, it was expected we would sit on the toilet to do our business.

Basically, the modern us overrides what is instinct.  Children still have those primative instincts and they go with them.  It is actually more comfortable to poo standing up, regardless of your age, because then gravity is working with your body.

I had a similiar problem when toilet training my daughter - she didn't want to sit. I think the problem is that they get so used to standing to do a poo when wearing nappies! After all, it is very hard to push ot a poo that has no where to go (when sitting with a nappy). I suggest that you go cold turkey - no nappies at all. If he still poos and it makes a mess you will be able to show him the mess and explain that it is better to do it in the tiolet. Being 3 now he can understand things a lot better than younger ones who are training.