I'd approach it as trying to clear up a misunderstanding, something like the following :
I'd like to talk about your last annotation on article X. I've agreed in the past to be presented as a second author in previous articles we've worked on together, because of A and B. For this article though, I've done A, B and C and thought it was clear I am the primary author of this article. I'd like this to be reflected in the author order, without annotations, like we did in previous articles. I’m sure you don’t mean to belittle my work, so is there something I'm missing of why you think this time we should add that we both equally contributed ?
I would do it this way because :
- It is non-confrontational approach, you’re not accusing him of anything, you’re trying to understand something weird. Make your tone reflect that, like you’re talking to a colleague about a weird issue.
- You’re highlighting what happened in previous articles, and how it differs with what’s happening now. With this you’re gently calling him out, you know there’s a double standard here, and you won’t just let it pass by.
- The last sentence is optional, depending on how “soft” you want to approach this and on how you think your colleague will react. If you find it too soft, you can change it too something more strong but still kind, something like this :
[…] in previous articles. I like working with you and would like to continue in the future, but I want my work to be respected. Do you agree with that ?
If he still wants to be named as an equal author, you'll need to be more direct, say you won't do that and re-explain briefly why.