[lambeausam]

Or it could be typical - "I didn't want to stop playing __ to take a potty break. Out of 12 foster or adopted kids, 7 of them did this between the ages of 3 and 4. All of the kids had mastered day-time toileting before turning three. However, it seems that once they become "toileting experts," they think they can hold it indefinitely or wait until the very last moment.

Just a suggestion - make a babysitting rule. He has to go to the bathroom as soon as he arrives. Then have your mil set a timer for 90 minutes and have him go again when the timer goes off.

Another thought - go in her bathroom with him and ask him if there is anything that frightens him. My 4yo is afraid of toilets that flush loudly.