Welcome, M., to the wonderful world of the toddler! Checking his hearing might be a good idea, to be sure that there isn't an underlying medical reason. My mom still tells stories about my sister and how the doc told my mom to set the baby down in one room - go into the next room and whisper "Does Linda want candy?" She came right into the room and said "Yes, I want candy!"
I wish I could tell you that you'll never worry about dangerous situations or your child getting hurt. It does come with the territory. You love your child, want to be the best parent you can be for him and want to protect him from anything and everything that could harm him in any way. It is a tall order. It is an awesome responsibility. Keep at it, Mom! You're doing great!
I can tell you if you find yourself calling your child from a dangerous situation that they will know your voice sounds different - that urgent tone - and they do listen.
Good luck with your son! Remember that he won't ever be as small as he is today - not even tomorrow. They are growing and changing every day.
D.