A crappy situation

Hi all! My 7 year old son is a dream kid in nearly every way. Kind, smart, caring, does well in school, etc. BUT he has a terrible habit of not going to the bathroom in time and pooping his pants. He's been potty trained for 4.5 years but at least once or twice a week will have far more than a skid mark in those undies. My wife and I were very patient and worked with him to make sure he was going more often, changed diet to make sure it wasn't constipation, started a probiotic regiment, and tried encouraging him and celebrating clean undies and going potty like he was a toddler again. But there was no real change. In fact, if anything, it feels like a slight regression as of late. Our frustration has recently been kicked into hyperdrive as he has been openly admitting his accidents are caused by him trying to hold it because he was having too much fun playing or watching a show. We have now began punishing that behavior by taking away privileges (no minecraft, tablet, etc) to show that it's far less fun to poop your pants and miss out on your favorite things for days rather than go sit on a toilet for 5 mins. Again, no change in outcome. To make matters worse, he sits in it and waits until we aren't paying attention to hide the accident and subsequent clean up from us. I know he's just trying to avoid consequences but the lies and sneaking only contributing to the overall anger and frustration with this situation. We are just at a complete loss on how to get through to him. This makes no sense to us and we feel like we've tried everything. Has anyone else experienced anything like this?