Holding Poop Until the Next Diaper

My 3y son was somewhat difficult to potty train for urine but we got there! He’s not a “ease into it” kid; given the choice he will always take the easy way out. We tried everything and ultimately just had to hold him in the bathroom to pee one day, and after many tears he got there.

We’re at a spot where he is fully daytime trained, and we use diapers for nap/bedtime. The problem is he saves all of his poops for when he knows the diaper is coming. The pediatrician said to not push it because of the risk of inadvertently causing poop withholding, but to be honest, my son can tell me where poop goes and does not go. He can tell me he needs cleaned up, the texture of the poop. He needs to go in the toilet, it’s time.

For context, I think he gives good effort, because he’ll sit on the toilet and “push” but only will pass gas. I fundamentally refuse to use food as a reward system, so that’s off the table, but we’ve tried a star reward system with no luck.

Any and all advice accepted.