It really depends. Our 3 year old normally does very well on her own but sometimes calls for me after she's pooped.
Then there are the times she's been in there forever and when I go to see what's up, I find her playing in the sink. She LOVES to play with the liquid soap and water. Empted a whole bottle once, bubbles EVERYWHERE!!! LOL!!!
Then there was the time she was too quiet in there. I got worried when I heard rain drops inside the bathroom. Got to the door and discovered that she had gotten the umbrella, opened it up and placed it on the floor. She was then pressing her finger agaist the fassett, aiming the sprayed water into the opened umbrella. I couldn't be mad because it was so darn cleaver... that and she had pretty good aim!
Children are naturally curious so if there is something you don't want them to experiment with just put the item where they can't reach. He'll be fine.
Sometimes you need to trust your child a little so they can learn to be independent.