On the other hand if he has removed references to other cofounders then that's a different matter.
Other than that, insufficient context details. Please elaborate.
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.
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.
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.
