His friends might have all been too drunk to drive. (My speculation.)
Good point. If so, perhaps Michael didn't want to ride with them for that very reason.
I know I tell my children if offered a ride by someone who has been drinking at all, they are to call me no matter what, no questions asked (they would definitely get a lecture afterwards, but I wouldn't want them to hesitate to reach out just because they or their friends have been engaging in behavior that I wouldn't approve of). Or to call 911 if someone's life is in danger due to drug overdose, alcohol poisoning, etc. Sure, there will be consequences, but that would still be much better than having to live the rest of your life knowing you could have saved someone's life and you didn't. There is no guarantee they will when faced with these types of situations, but I just want to them to know nothing is worth losing your life or your friends' lives.
Another thing is Michael isn't of drinking age. Maybe he thought if his parents found out, they would complain to the person -and/or his/er parent(s)- who furnished the alcohol and they would get into trouble, and thus he only texted his brothers.
I just feel so terrible for his family. For one of his shoes to be found, followed by the other shoe, it's like they are being subjected to some sort of cruel, prolonged torture ...