Inability to commit

My ADHD husband frequently refuses to give a clear answers.  A much too common example is when someone invites our family to do something, he will totally not respond or say he has to ask me, then he completely forgets about it.  I feel like it makes us look incredibly rude.  He's the social one so people frequently text or ask him rather than me, and I might not hear a thing about it until a day or two before when they've had to contact him AGAIN about whether we're coming, at which point I may have actually made other plans, and then he decides he wants to go, and I'm irritated at having to cancel MY plans AND having something entirely different sprung on me.  I laughingly (cringingly) tell people all the time that they have to group text us so I know what's going on, but nobody does.

Our oldest son (18) is taking a gap year before college.  He works full time with his dad and doesn't have his own vehicle yet, so we let him borrow ours.  But if he tries to make plans with his friends - if he needs to use the vehicle or change hours at work - my husband won't give him a clear answer even if the plans are time sensitive.  His girlfriend will say, "Just ask him if that's a yes or no."  My son has started recording the nonsensical answers his dad gives him... even when clearly asking whether it's yes or no.  He's a respectful kid and a hard worker, but he is quickly reaching my level of bonkers with this because it is a constant thing he now encounters at work and at home.  

I don't know what to do.  I can't say anything that makes a lasting impression because my husband will always get defensive and say "I just forgot!"  My son doesn't want to keep bringing up a request because he recognizes the vehicle and time off is a privilege.  He's fine with the answer being NO, he just wants to be able to let other people know so they aren't left hanging.  I don't know how to rectify this.  I've been dealing with these kinds of frustrations for a long time.  My teenage kids are just now starting to share in the experience.  How do we deal with this without me nagging or the kids feeling like they're being disrespectful by having to ask time and again to get a clear answer?