"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 yoursel

"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.