Perhaps he has seen you use your key to open the door, he may be inquisitive, looking for doors to open. Again, perhaps the stethoscope he has experienced in medical exams, he may seek to know more than can merely be seen or touched.
These are guesses, the quality of you being a mother is not a guess, it is a fact of your life.
There will always be worries, but worries would not exist without love for the worried one.
Fear not, you are doing well, adjusting to the changes, the growth, the lives of your children should be a joy, at least most of the time.
Enjoy the ride.