Telecom reminds users whether they can receive SMS prompts when the other party shuts down.

Unless he downloaded something, I don't seem to have heard of it (maybe I'm short-sighted), or you can give it to him.

Send short messages

Tell him to call back when he turns it on. As long as he turns on his phone within 48 hours, he can see the short message you sent him (generally speaking, if he turns off his phone, he can receive the short message within 48 hours).