Why would they have to promise him anything? What's he gonna do? Go pro? Good luck with that. And if he transfers, he'd be sitting this year anyway, so there's no point in that, either.
Here's what they told him: you can play this year, be out of eligibility, and maybe get some garbage time after Jennings, Calloway, and Palmer are done for the day OR you can redshirt in 2019, come back next season, and compete for the abundance of available minutes left by the departure of Jennings and Calloway and end your college career on a high note. Even if he doesn't start, he plays a lot.
It's a no-brainer. The kid would have been an idiot to turn down the redshirt option this year, unless he's just tired of football. There's no need to promise him anything other than a chance to compete. No coach is gonna promise more than that anyway.