Johnson was a grey shirt so he already lost one year off the clock before he got here. If he transfers, that leaves him only 2 years left without playing a single snap of college football.
You do not lose a year of eligibility with a Gray Shirt. You are not enrolled full time so the clock does not start. IF Mike Johnson transferred at this point he would sit out another year. And then have 3 years of eligibility remaining.
Mike Johnson will be classified as a RS Freshman this fall. 4 years eligibility remaining.