Maybe get him a potty to sit on and that way he will get used to it and if he goes to poo on it then just praise him like you have been.

I had a potty around the house since Kimberley was 1 but she wasn't tt until 2.5 she came home from DH's cousins one day and that was it she wanted to use the toilet. All the months of us trying and she did it all on her own.