recording conversations: childish or helpful??

So I am at the end of my rope this weekend with DH's inability to recall pretty much ANY conversation we have, either just the two of us or with the kids.  My oldest son turns 18 tomorrow.  He had prom last night, graduates from high school next Sunday.  He is VERY full of himself.  About two months ago, this child started teeing up the "when I turn 18 I can do whatever I want" conversation.  I mostly blew it off with humor--like "yep-good luck with that" type of thing but the more he brought it up, I had enough.  This same child leaves for a gap year in Costa Rica in September.  He is a beach life guard, manages a beach all summer.  He has also had issues with drugs and alcohol.  To say he does not have the best judgement in the world is an understatement.  He has a 1:30am curfew.  And every freakin' time he goes out we have to have the curfew conversation again.  So DH and I agree that this child will have a 1:30 curfew from NOW until he leaves for Costa Rica.  And both me and hubby have this conversation with the child.  In fact, it is DH who delivers this information.  And would you of yesterday, DH has no recollection of this conversation at all.  Convenient, huh.  So I told DH yesterday that we will have this conversation with the child AGAIN and this time I will record it so that when either of them "forget" this information, I have a record of it.  Second thing DH "forgot", also related to this child.  Child wanted to go with his friends to a "prom house" in Lake Geneva for a few days.  And BTW, a "prom house" is not a thing.  So no.  And again, DH and I had this conversation, DH agreed that he had never heard of such a thing, etc.  And then today the child is mooning all over the house because "all of his friends are at the prom house" and lo and behold--DH DOES know all about prom houses, they had them when he was in school, he has heard about them all over the place.  Um...what?  DH did not go to prom, he currently has no friends, and spends every moment he is not working parked on the couch watching either youtube videos or Storage Wars, so where he would hear about what's hot in high school is a mystery to me.  

I know that others have similar problems with selective memory with their ADHD person.  Couple that with the "I know everything about everything" even with a LOT of evidence to the contrary and it is crazy making.  I start to feel like I am the one who can't keep stuff straight.  DH and the 18 year old have such an odd relationship-I am very anxious for the child to get away from the lousy influence that is his father.  Most things that DH and I discuss TOGETHER, and then present to the child, DH words it with some version of "your mother wants you to..." or "mom thinks and I agree".  Really not helpful.  The child tells me all the time that I am too hard on DH and that I am so bossy that DH just goes along with whatever I say.  I suspect DH's "selective" memory and desire to appear in a positive light EVEN if it means hurting OUR relationship is part of the ADHD impulsive, whatever is easiest in the moment, avoid conflict at all costs thing but boy--not mature at all.  And really hard to parent that way.

I am ready to record conversations that we have with DS so that NEITHER of them can claim that a) the convo never happened or b) the content of the discussion.  Thoughts?  I have to get some sanity here.  And what usually ends up happening is that DS pushes back when DH is out of town or working late or has gone to bed already and then once again, it's ME that is the enforcer.  SO tired of that dynamic.  

Any help or advice on dealing with the ever-so-convenient selective memory would be much appreciated-