How different will he be under meds?

My partner was diagnosed many years ago, and mentioned it 'in passing' shortly after we started dating (and living together) 20 months ago. I just figured that explained why he bounced off walls at times. However; his stress, anxiety and anger, while initially explained by stressful life situations, caused me to eventually wonder if something else is going on. I have only recently started researching - and found he is a classic case of ADHD/anxiety. He is highly functioning, has got a lot of self management systems in place, but in some ways is quite immature, cold and clueless.

I believe he didn't realise how big an issue it was for our relationship. I am clear that if he wants to stay with me he needs to try meds. He didn't want to take them but recognises I have run out of steam. I don't have much patience left - I'd worn myself down being "supportive" during the 'temporary' stressful times, little knowing that ADHD was at the core. I know now that he had tried Ritalin before without much success.

I am wondering, and asking all of you experienced partners, what can I reasonably expect will happen when he is under meds? I understand it will take a while to find the right drug and dose, but I don't know what is hard-wired and what will soften/change?

He is impulsive and erratic (I don't mind) has a lack of empathy (I find this hard) is unable to be 'present' (ditto). He has a low self esteem and is constantly worried about being 'wrong'. I'm worried that the only thing that may change is the one thing I don't mind - his erratic nature!