8 year old peeing pants

my son lives with his mother and her parents.  i get him every other weekend, and a month in the summer.  he has had a problem with wetting his pants for about three years.  it is only when he is playing, he says he doesnt want to stop.  his mother says she thinks he has a medical problem, and keeps using that for the reason he does it, but hasn't taken him in to the doctor yet.  i have an appt to take him in this week, ( i have him for the month now).  the problem i have is that he does this all the time at his mom's, my other two children tell me, when he is here, it happens once, I set the expectations of behavior, and it doesnt happen again.  we go through this every time he comes over.  he told me he doesn't want to stop playing, thats why he wets himself.  since this is happening primarily at his mothers, should i see this as a warning sign for something i should be concerned with.  how should i not only handle it with my son, but with his mother.

[anxiousmomtobe?]

First of all, you shouldn't question the other kids, that is putting them in the middle, and I know from being a kid in that position, you tend to say what you think the parent wants to hear.

Secondly, rule out any medical cause.

Thirdly, sit down in a neutral environment, with you ex (the kids should not be present) and calmly discuss your concerns.  Make sure you are respectful, she is their mother and is doing the day to day grind of homework, dentist apts, etc.  I am sure you are both loving parents who only want the best for your kids, try to focus on that and work as a team.

Finally, it's really not that uncommon for 8 year olds to have an accident now and then.  If he seems to be doing well otherwise, I wouldn't fret too much.

[Cleveland Mom]

Rule out medical problems with your son's health care provider.  In a child your son's age, toileting accidents may be representative of regressive behavior.  When children are under significant stress, it is common for them to regress to an earlier stage of development i.e. toileting accidents, thum sucking, clinging to adults.  Do you believe your son is under any unusual stress?

If not, tell him he cannot play until he urinates, if he has not recently done so.  If he refuses to toilet himself, he cannot play, he must sit.  During his play, monitor him for signs of having to urinate, bouncing, grabbing privates.  Tell him he must toilet himself if he wants to keep playing.  If he refuses, then remove the toy he is playing with and tell him he must toilet if he wants to play.  
Best wishes...