For a redshirt freshman who transfers and has to sit out, "redshirting" doesn't matter because you can't redshirt twice. So either look at it as he took a year of eligibility at Mississippi State and will redshirt at Illinois or he redshirted at MSU and will take a year of eligibility sitting out here. Either way, he enters 2021 as a "3 years to play 3" player.
If he were to get a waiver for this season, he could play immediately, but that 5-year clock will still only have 3 years remaining in 2021. Barring injury, he won't be needed this season, so he won't play, which means none of that stuff matters (waiver, redshirt, etc). He'll simply enter training camp in the summer of 2021 with three seasons left on his clock.