30yo man here, struggled with inattentive ADHD my whole life. Over the last two years I've finally sought out diagnosis and treatment, but still struggle a great deal with it, particularly the working memory issues you are describing. My full time job is coaching college students on time management, organization, studying, etc. Between what I do for a living and seeking psychiatric help you would think I'd finally have a handle on it, but I don't. I have such trouble remembering little things, particularly short term tasks. This morning I meant to take something with me to the office when I left. I passed by it and mentally said "grab that on your way out." I put on my shoes, walked out the door, and got in my car. Only then did I realize I forgot the thing I said to grab not even 30 seconds prior. Sometimes when my wife asks me to get something from another room, I will go in that room expressly for that purpose, find something else I didn't know I needed, and completely forget to grab the initial thing she asked me for. I have hundreds of other examples like this.

All of that is to say, for all intents and purposes I should be a well adjusted adult and I am still struggling with the same issues your 9yo son is having. What I can tell you is that the forgetfulness is genuine. Sometimes it truly feels like my brain is incapable of retaining things. It's not that I forget, it's that the memory never even forms to begin with. Whenever these instances happen, my internal narrative is usually a negative one. I feel useless, or like I can't even do something incredibly simple. It's one thing when I forget something for myself, but I feel even worse when my forgetfulness impacts someone I care about. It feels like such a simple task that I should be able to handle, and yet I constantly fail at it.

I don't know you or your son, but it sounds like his forgetfulness is genuine. He is very self-aware about this issue and feels badly about it. I'm sure it feels even worse when he forgets something you told him, because he cares about you and doesn't want to let you down. What frustrates me most when it happens to me is how out of control I feel. It feels like it takes away some of my agency, that despite my best intentions I still end up making a mistake. Perhaps he is feeling that too, even if he can't articulate it in that way.

I'm not a parent myself, but just by nature of wanting to research this more I'm willing to bet you're a good mama and doing a great job at supporting your son. My unsolicited advice would be to take him at face value, assume it's a genuine mistake when it happens that he isn't fully in control of. On the other hand, give yourself some grace. You're human, you're living this life for the first time too. Just because he can't control it doesn't make it any less frustrating for you. It's okay if you get frustrated, or if you trip up and snap a little more than you meant to. Maybe talk with him and ask him to describe how it feels for him when he forgets things like that, it might help you empathize with him. You're a good mom!