Son 3 years and 8 months and will not pee or poo in potty

My son is perfectly happy to pee in his pull ups.  When I put his big boy pants on him he will hold it and when I ask him if he needs to go potty he says no.  He will sit on his potty but won't go at all.  I get frustrated and put his pull upl back on him and he goes pee right away.  I have even let him stay in his wet pull up hoping he won't like the feeling and he will start telling me he has to pee but he doesn't.  The pull up would be so wet it practically falls off of him and he still never says anything.  In fact I tell him to come to me so I can put a dry one on him and he tells me no.  I have tried taking his toys away and not letting him watch his favorite cartoons telling him if he pees he can have them back and still nothing.  My daughter is 12 and when she goes to school my son wants to go too. I have told him he goes potty in the big boy potty he can go to school too.  He says ok and goes pee in his pull up.  I am at a loss I don't know what else to do.