If he prefers the toilet - let him. Friend's DS was PTd just after turning 3 and from the start he used the big toilet and a stool for standing up wees. He didn't get on with potties at all. Because he was tall enough, he managed the toilet fine.
I know he is still struggling with going for a poo - like many children do. My tip for that - make sure if he is sat down he has something firm to place his feet against - we found this certainly helped with DD>