You did the right thing. Follow this up with a police report if you think your son will be ok.
Disconnect, emotionally, from any concern for any one else's opinion about your choice to do this, including what the police may "think".
Disconnect from the reaction your ex will most likely have to this toward you. You do not have to play his game anymore. You make the rules now and it does not involve being "afraid" of him. (Yell that ten times, like a warrior in the mirror each day!!! grrrrr)
If he "retaliates" in any way toward your son, you will have supporting evidence. (again, you will have to know your son will be ok, and you need to make that call).
Your ex will either stop this behavoir, or bury himself with his egoic reactions. Either way, you will ultimately protect your son.
My exN was driving my kids after "happy hour" everyweek. I emailed him telling him to stop (did not accuse him of being drunk, though, as he may not have been), forwarded it to my lawyer as well, and he stopped. He is not stupid.
So let this be your first (or one of many already?) battle cry that you will protect your son to the ends of the earth.
YOU are powerful and loving. An unstoppable combination.
Sending you lots of love and strength.