A [cofounder] is a subset of [founder].

On the other hand if he has removed references to other cofounders then that's a different matter.

Yes this is the case. He's now claiming that he was the originator for the idea which is far from the truth.

You have to engage with him in a positive manner first, ask him politely to correct the accuracy of the statements. If he doesn't within a few days then pressure him again, perhaps mentioning your desire to 'not' involve a legal team. That should do it. Of course if he reacts badly it make swiftly become a good idea to involve lawyers as he's really stepping over the mark.

To clarify is he calling himself "a founder" or "the founder"?

Being part of a team and calling himself "the founder" is a dick move and you need to talk to him about it.

If he is calling himself "a founder" you need to chill. There is no difference between a founder and a cofounder. If it is so important to you start calling yourself a founder too.

People like this piss me off - and I agree it diminishes the other founder(s?) positions if not technically then at least in perception.

I know you should probably talk to him/her, but if they are this arrogant you are likely only to get the words they think you want to hear, and no action. Worst case, they realise that you've rumbled them, and they go all out to raise their profile at the expense of yours.

If I were in your shoes I'd copy the tactic. Call yourself the founder too. DON'T make things up, but state the genuine claims as to what your contribution is/was to the founding of the company.

This way you let other people make their minds up. As long as you can support your contribution claims, then that's what will eventually win through.

How about just ignore him? How does this affect you? People lie about their resumes all the time.

Don't want investors in my next company to ask why one of my cofounders gets to designate himself the founder while I'm labeling myself a cofounder.

"a founder of XYZ" has the same meaning as "a co-founder of XYZ", but it is potentially more confusing.

Either ask him to use co-founder or simply call yourself "a founder" as well. If he's using "the founder" that's just not cool, so tell him so.

Quit bitching. Who cares. People evaluate you on what you do and what they see you do, not on what others do. If people misunderstand because someone else misinforms them, that's not your problem. You can't fix every miscommunication.

Possibly the same thing you would have done if he/she had done it prior to acquisition.

Other than that, insufficient context details. Please elaborate.

I'm starting to incubate my next company and don't want to have to explain to investors why one of my cofounders is calling himself a founder when I'm still labeling myself a cofounder. Additionally, he's phrasing his work profile/resumes etc to indicate that he was the originator for the idea of the company.

What's the difference?

Did you talk to him? Ask him why he is doing that?

Exactly. Ask him first, then if he doesn't see a problem with it after explaining your side, write an entry on the issue somewhere so that you can point people to it later. Blog post, etc.