We had some similar issues with Jett. For instance, being SO INVOLVED with his video games that he would go in his pants. And not notice. I don't know HOW he didn't notice.
Major issues with him wetting himself at school, and at night. Actually getting up - and urinating on the floor in the bathroom - or worse - in his bedroom (carpet...) BM complaining that he was so afraid of husband, he would poop himself when he came in her door (um, wouldn't you think it would be when we picked him up? LOL). Doing laundry, and there was a nice big brown present for me. (SUPER UGH.) We tried making him clean up, making him wash his underwear, making him take breaks from his games, arranged with the teacher that he had unlimited bathroom passes. Nothing worked. Stress made it worse.
We did discover that it was MUCH worse when he came to us from BM's. Especially noticed this after we got residential... He'd go visit on a Wednesday, and that night we wouldn't sleep, waiting for him to get up... So we could pop out of bed and guide him to the toilet.
And then we weaned him off the Concerta, when we changed his pediatrician from BM's general doctor. It turned out he was CRASHING - and could not wake himself. Still had some issues at school, but not nearly as bad. STILL had issues after visits with BM. But overall, things were better. After about a year, suddenly this wasn't happening after visits either. Phew! ...And then he mentioned BM was out of Concerta (and our doctor wouldn't write a Rx for her), and why did he have to take it when he was there? (WOW...) We also quit making a big deal of it. He still had to clean up after himself, but if he wet the bed? OK, go wash your sheets. Onyxx took a little longer, shaming him, until we finally told her - STOP - it's stressing him out and that makes it worse!
So to begin - before he was on the Concerta - it was the stress, I'm pretty sure. And then, when he was ON it, BM kept having the doctor up the dose (9mg > 27 mg > 54 mg within 6 months), so his poor body was trying to keep up.
Add in the fact that he's developmentally delayed, and it's just a huge recipe for disaster.
Every now and then he will wet himself. He just cleans himself up and goes on. But he's better at noticing.
Also, FWIW - I read an article recently about kids with ADHD and soiling issues being pretty common. I wish I could remember where.