I would not be reporting this to the Police, anonymously or otherwise. The potential ramifications for something that was extremely foolish as well as illegal, but is over, could be far reaching. Driving and owning a car is incredibly expensive for youngsters at the best of times, but if he has a black mark before he even starts, your son's life could be limited a great deal, both socially and in terms of work.
I'd be having a serious chat with him about what "someone" believe they saw, and make it clear to him the risk he would be taking, and the cost to him and his future, let alone any other road users if anything had gone wrong.
What a selfish, thoughtless father he has, to do such a thing rather than get a taxi. Your son is not responsible for his father, but needs help to see that he has to take responsibility for himself and his actions, and that they have consequences. He got away with it this time, and how fortunate that is for all concerned.
Scare the shit out of him, but don't involve the Police - no-one was hurt, thankfully. He made a mistake, he used poor judgement, and your ex should not have allowed it, let alone encouraged it - selfish twat. Give the kid a chance. The great thing about being human is that we have the ability to learn, so point out the risks and potential cost to him, he's old enough to understand the implications. Then tell him that if he does anything remotely similar ever again, you will contact the Police.
I'm sorry, and realise it must be scary for you, but Police involvement won't change that. As for the ex, tell him to book a taxi if he's going to be drinking around your son.