It's well worth having a focused conversation with him, as close as possible to when he wakes you up (eg the next morning so it's fresh in his mind).
"Bill, please can we talk about the fact you woke me up and started talking to me at 3 o'clock this morning. There's nothing worse than sleep deprivation, once you've woken me up and started chatting away to me, I can't get back to sleep so that's my night ruined for me. I really need you to keep quiet once we've said goodnight, it's becoming a real issue to me, I can't operate on 2 hours sleep a night".
see what he says when you say it how it is,
if he takes it on board, then you know he cares about your needs.
if he flat refuses to make any effort and you see no improvement then it's goodbye, nice knowing you, sorry you don't give a shit about me.
I'll be telling him an anecdote and he clearly isn't listening and he'll get distracted by something pretty standard like a tree (I mean something that he could easily wait a few seconds to remark on but instead will just cut in on the story instead) so I frequently don't tell him news or anything that will take much concentration from him.
re: interrupting you to talk about a tree lol, I'd pick your battles and get the night time situation sorted out first, that's most important. Once that's under control, you can then start saying "um Bill, I was saying something when you just stopped me in my tracks to talk about that fine 200 year old oak, but can I finish what I was saying in future and then we can talk about the oak, is that OK?"
if he is ADHD, then it will take a bit of time and patience to help him adjust his impulses, but if you think he's worth it in other ways then give it a try and see if it makes any difference.
also, bear in mind your own behaviour. If you're telling him an anecdote, you need to get to the point pretty quickly, sorry to say it like this, but if you're rambling on and that's taxing his powers of conversation, don't expect him to sit there patiently while you get to your point, you need to adjust your communication style too. Give and take, and all that!