Potty Woes - How to Recognize Sensation?

My 4 yr old son was having a great run of peeing and pooping on the potty for the past few days of spring break. Today he pooped on the potty at a playspace, but then I smelled something and he had def pooped again a few min later. We changed his underwear (put him back on potty to make sure all done) and then maybe a little later he def peed himself. The thing is that I realize that he was great at going on the potty, bc I was forcing him to go every 30 min -1 hr. He doesnt get the senstation, nor is he bothered by being wet or sitting in poop.

Any ideas on how to get the sensation down?