For some presentations* of ADHD, the key is repetition, repetition, repetition.
Accepting that his failure to pay attention is how his brain works, rather than laziness or disrespect, was a key moment for my relationship with my ADHD son. It made it easy for me to not become angry when he "ignored" me. It's tedious to make the same request more than once, to remember to say "are you listening, this is important" before making a request, to check on whether a request was completed (and completed fully/correctly) -- but it's just a process I have to go through because he isn't able to focus easily. That's just who he is.
Years of patient repetition from me has led to a pretty responsive child who does his best to helpfully respond. Still not perfect, but he's willing to try because he knows I won't be getting angry if he isn't perfect; before we figured this out, I'd become quickly frustrated, and then he'd feel inadequate and frustrated.
Once you have a diagnosis, be sure to share it with his teachers. Most are very understanding of disabilities like this, and are willing to give a little extra reminder or accommodation (within reason) for ADHD kids.
* My experience is with primarily inattentive, rather than primarily hyperactive, so my advice may not be ideal for all families.