Drawn into lies

Background:  My daughter (9) is diagnosed ADHD.  I strongly suspect my wife has ADHD, and we both suspect our son (14) has it.  One of our son's symptoms is the frequent and often pointless lies.  ("Did you start the laundry?"  "Yes."  It's not started when I go down a few minutes later.  "I took my cell phone out of the case and accidentally dropped it."  "OK, I broke it on purpose to get a better phone.")

On Halloween, my wife took our son to the ER.  She said that he told her he bumped into a tree and a branch fell on his hand, cutting it.  My wife was upset about the amount of blood and the fact that he kept trick or treating for hours without telling us--or doing anything beside putting a band-aid on it.  I suspected that it was not a tree and expressed some disbelief.

Last night, my son told me that it was not a tree.  First, he told me that friend 1 had been dressed as Link and cut my son's hand while he was fooling around with a replica sword that was part of the costume.  He then changed it to say that friend 2 had taken the sword from friend 1 and cut him while he was playing with it.  Then he tries to get me to promise not to tell my wife or our daughter.  Our daughter is in the next room, so this was not time to do this.  I did not make any promises--If I made this promise and it does come out, it would mean that I was lying, giving him reason not to trust me. He already distrusts his mother, and for good reason.  She makes frequent promises she does not keep--and she should realize she can't guarantee--such as "we're going to Disney this summer!"  Years of the Disney promises have taken their toll.

There are two reasons not to tell my wife.  First, she would probably get very mad.  Her fights with the kids do not go well.  Imagine two people with both the impulsivity and false memory problems of ADHD going at each other.  He has also accused me of being a "coward" for not standing up to her.

Second, I want him to be able to confide in me when something really bad happens--like an STD.  If I tell her this, than why should he confide in me about something serious next time?  He previously told me that a friend tried to get him to vape and then tried to make me promise not to tell her.  I have not told her about that.

Perhaps the best approach is to only mention this if there are related problems, such as further injuries or issues with friend 2?  But then I get into the "Why didn't you tell me this" argument.

I asked him why he told me this and he said he needed to get it off his chest.

This took place in the context of our daughter having a fit over not getting to have candy because she refused to do her homework.  (The teacher gave her a homework pass for Halloween--but she left it at school, of course.  Keep in mind that she will miss two days of school next week while traveling with my wife, so she could really use a homework pass--if she does have one--for one of those days.)  My wife was out for the evening.