Feeling defeated with potty training

So I could really use some advice- my son will be 4 in February and we’ve been consistently potty training since a couple months after he turned 3 years old. He does really good with peeing. We keep him naked from waist down all day and he’ll go pee fine with no accidents, but when it comes to poop, he will literally hold it in until he has a pull-up or underwear on for bedtime or when he leaves the house. I don’t know how to get him out ofthis cycle. I’m not sure if there’s a way to actually get him out of it or if it’s just something that has to click naturally for him. it’s just becoming an issue because the preschool hes in doesn’t mind that he wears a pull-up but they are getting fed up that he keeps having #2 accidents- since he wears a pull-up or underwear there he feels like he can just poop himself instead of holding it in like he does when he’s naked from the waist down. Really don’t want to see him get kicked out of preschool over this because he is speech delayed and also lacks social exposure so I felt like this was a really good thing for him!