Potty Problems

Hello,
I have a 4 1/2 year old son who is pottied trained but gets lazy and
doesn't want to poop in the toilet. He rather go in his pants and not
say anything about it.

I have tried everything to get him to stop doing this. I have punished
him, by spanking, time out, taking the tv or playing time away for a
couple days, I even try just talking to him.

He doesn't do it to get attention because he gets so much attention.

Does anyone else have any ideas on how i can get my son to stop doing
this?

Thanks,