My son is basically potty trained he knows how to sit on his potty to poop and stand up to pee but…

My son is basically potty trained he knows how to sit on his potty to poop and stand up to pee but how do I break him out of holding his pee ....when i ask him do he have to go he says no but when i make him go he let's it out i tell him it's not good to hold your pee and poop and btw hes 3 years old any ideas or help?