Hello and Thank You for using Just Answer. I am sorry to hear about the difiiculties that you are having with your son and his bathroom problems. To rule out any medical issues, you may want to have him evaluated by his pediatrician. If there are no medical reasons for his behavior, then you must focus more on your son's positive actions. At 3.5 years of age, he is very well aware of his actions. Set up a reward system for every time or every couple of hours that he stays dry . Keep him in regular underwear. Make sure he is being taken to the restroom on a regular basis. He may become so involved in playing that he forgets to visit the bathroom until it is too late.
Use a sticker reward system. When he has a great day or half a day of being pee and poop free, then you give him a sticker on a chart or on a calendar. After 10 stickers (or you determine the amount), he can be rewarded with some treat or activity. But you must make a REALLY big deal about when he pees or poops in the toilet and not on himself. This process may take a while because he knows that he is in complete control of his behavior. Your goal is to try and shape and mold his behavior by focusing on the positive actions and not on the negative which will not help him improve. The nursery must be willing to work with you or else find another nursery.
If you need more advice, just ask.