Well there's this little pesky thing in the MIT license:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
That's what he was asking for, a mention in the credits.
Since he eventually got the credit, your unfairness argument better build entirely on the damage the creator suffered by the delay of his credit.
He got his name in the credits. The question was if he is owed anything else. The contract he created says he was not. I’m simply suggesting he might need a different contract.