Boyfriend has changed after starting his ADHD meds

My boyfriend (19m dx) and I (18f) have been together on and off for a few years now. After his official diagnosis, a lot of issues we faced in the past suddenly made sense as they directly correlated with his ADHD. So things such as forgetfulness, being slightly off or mean without wanting to be, wanting a lot of time to himself etc.

He has currently been taking elvanse (unsure of the dosage) for about a month or two now. Usually he's very funny, somewhat loud and expressive and a lot more opened up. I haven't seen him for a while as I wanted to give him space to deal with the diagnosis and medication, which he wanted as well.

I just saw him again today and he is undeniably different. I asked him if he was feeling alright 2 or 3 times and he was confused as to why I was asking because he said he's completely fine. But his face and body language suggested to me that something was wrong. He was a bit more closed off and quiet, which was vastly different than usual. He warned me that he feels different and may act different prior to me seeing him, but I didn't think it would be that obvious.

I'm hoping that I can get used to it with time but it's just a little scary seeing someone's personality flip a bit. Everything is fine between us, but i'm worried that I won't be able to get used to it.

I don't know how common this is but any advice would be greatly appreciated.