Lea, this is really, really common. Having lived in Asia for a while, personally I believe that sitting is a very unnatural position to poo in, squatting is much easier, so no wonder so many kids take a long time to learn to poo sitting. There are some things you can do to help however.

Firstly, make sure he is drinking plenty of water so that there is less liklihood of constipation - hard poos can hurt and will set you back in the process. Also if you are using a potty, you might be able to help him be in more of a squatting position than sitting to start with and that might help. Both my boys started using the toilet straight off without the potty step (they are big imitators so DS1 wanted to do what mummy and daddy do, DS2 wanted to do what DS1 does!). On the toilet, if they have a stool at the right height for their feet to rest on, a good position is with feet flat on the stool and leaning forward a little. Maybe trying that would help also.

And the rest really is just patience. It will happen - it will probably surprise you and happen out of the blue. It's just a matter of gently persisting and waiting. GL.