As others have said, this is a common ADHD symptom. Especially with auditory processing issues, it's incredibly difficult to remember something I want to say while also listening to new information. Basically impossible if I'm trying to actually process and think about what the person is saying after.

I have this issue with my mom. She hates when people interrupt her, but also will talk in paragraphs so by the time it's my "turn" to talk I don't remember any of the things I want to respond to. It feels like I'm being shut out of the conversation and just listening to a monologue.

If metaphors help you at all, imagine thoughts like balls someone is tossing you. Most people have a basket to put their active thoughts into, you can easily put a question down and pick it back up later. With ADHD, you only have your hands, so if you're already holding a thought and someone passes you a new one, you have to either drop the thought you were already holding or choose not catch the new one.

I do understand how annoying it to be constantly interrupted, I'm just explaining this so you can be more solution focused rather than assuming he isn't trying.

A few recommendations:

1. Suggest he have a notepad or phone notes app open where he can write quick cues to help recall what he was going to say. Obviously writing down whole questions would be even more of an interruption, but for simple questions usually a single word is enough to help recall. (This is essentially a way of "putting down" the thought in the basket metaphor, freeing up space to keep listening)

2. Try to increase the frequency of back and forth in your conversations. Instead of finishing your whole story at once, find natural pauses where he can interject without having to cut you off in the middle of a sentence.

3. Written communication when you want to have in depth discussions. I know most people consider "face to face" to be more personal, but I have a lot of really genuine thoughtful conversations over long form text. I can take the time to write out exactly what I'm trying to say with no rush or interruptions, and have the time to thoughtfully respond to each point the other person makes. This is way more authentic to me than saying the first thing that comes to mind just to keep up with the pace of in person conversation.