Potty training

I need some advice about potty training 😩. We first tried training underwear with my 2yr9mo old but he used it like a diaper. So then we said ok let’s just put pants on, no diapers/pull ups/underwear, so that when he goes he’ll get wet. For the past 4 days we’ve had completely dedicated time to focus on potty training. The first day we didn’t get him to go until the last peepee before bedtime. The second day be only had one accident! Then the third and fourth he completely regressed. He sits on the potty like a chair. He’ll hold it for HOURS until he’s squirming, but won’t just go on the potty. He wets himself and I’m starting to be convinced that he doesn’t even care. We’ve been practicing for weeks before this marathon weekend to get him used to it. He’ll sit on the potty playing with a toy or watch tv for so long without any action. Then he’ll get up and wet himself 5 min later. I tried m&ms as a reward. He is a typical 2 y/o to get him to do something has to be his idea. I go back to work tomorrow and he’ll be back with my MIL who watches my 1 y/o too so to expect her to have this level of commitment to keep him on track is too much to ask. Thank you for the tips!! 🙏🏼🙏🏼🙏🏼