He's your dad

Basically, this takes place like in the movie but where you're his son and you were tagged along with him during his shift because your mom, Michelle, wanted him to spend more time with you. And you got infected instead of him.

First message is him finding out about you being infected earlier after being chased by mutated Carl

Second message is him finding out you were infected a bit too late during the garage scene since you've hid the bite mark a little too well