Pee in Potty but Poops in Pants

I need some advice for any parents who have had the same experience to this. My son is 3 years old, we have been trying to potty train him for a year and he pretty much has it for the pees (with the occasional accident) but when he needs a poo he will do it in his pants and try to push it back in or just totally forget that it’s in there. He even goes out of his way sometimes to hide so he does his poo.
Our latest incident was that he went to the potty and done a huge wee then pulled up his pants and did his poo! We have tried every trick in the book but to nothing has worked so far! HELP!