Hi there. One thing I wanted to chime in with is that he needs a medical evaluation. This sounds a bit like classic signs of encopresis which does plague some kids. Kids of this age generally do not want to have accidents. And encopresis is such that they do and have a hard time with the whole bowel movement experience. They soil underwear commonly. Basically, it is long standing constipation, straining to go to the bathroom. Kids with this condition don't always know when they will go because they often strain and feel like they have to go a lot. It also hurts to go. They DO have bowel movements though so parents don't realize this is going on all the time. My son has had encopresis and had regular bowel movements! Passing very large stools, pain when passing it causes 'holding'. Poor kids, right?! Adding fiber and fruit helps, my son drinks a glass of grape juice daily. I would definitely talk to your doctor about this condition to make sure this isn't what is going on.
The doctor will do a physical exam and may take an x ray. Sometimes the colon is entirely filled up! There are other further tests they can do for it. And unfortunately, treatment often begins with a full enema. My son has had one -- fun times! But he didn't complain. It's a bit of a relief. And I tell you, until we ended up to the emergency room due to stomach pain (due to fecal impaction), we had no idea he even struggled with constipation or had this issue.
So, see if you can schedule an appointment with a pediatrician you trust to evaluate this. Perhaps on one of your weekend visits as many doctors do have Saturday hours.
I'm very sorry you see him so infrequently and his schedule is such a wreck due to his mom leaving. That's very sad! Try to be as stable as you can for him and make him feel safe and secure. Keep routines going strong whenever you are with him! good luck
It sounds like your son is living in a pretty unstable environment for most of the week- things like accidents and lack of hygiene can be attributed to having a lack of stability- and he is only ten. I would encourage you to attack this more from the angle of his emotional well being rather than just a behavioral issue. Are you able to get him to go see a counselor on a weekly basis? Is his grandmother the primary care giver since his mother is gone? Is it possible you could take him on more of a full time basis if needed?