I'm sorry this has happened, but slow down and take time to understand these concepts.
Redshirt means that a player didn't pitch a single inning or have an at bat all season, and thus the season doesn't count against his eligibility. So, the coach did not lie; he gave your son one inning, meaning that he has used a season of eligibility and cannot redshirt. The time to have asked about redshirting would have been before that inning.
Players were always allowed to transfer from a 4-year to a JUCO and then back to a 4-year without sitting out. That hasn't changed with covid. What changed is that now players can transfer one time from a 4-year to another 4-year without sitting out.
This thread from last year is about D1, but seems similar to your son's case, and had lots of good advice:
https://community.hsbaseballwe...to-go-into-portal-d1
Good luck.