https://www.google.com/amp/www.techadvisor.co.uk/how-to/mobile-phone/has-someone-blocked-my-number-iphone-3476005/%3famp
What happens to a blocked text message?
Texting someone who's blocked you works as you would expect. The message sends as normal, and you don't get an error message. This is no help at all for clues.
If you're using an iPhone yourself, you used to be able to get a clue as to whether someone has blocked you by sending an iMessage. The iMessage would attempt to send, but after a couple of minutes it would resend as a text message that would never be received by the person who blocked you.
Now, though, Apple has updated iOS so that (in iOS 9 or later), if you try to send an iMessage to someone who blocked you, it'll immediately say 'Delivered' and remain blue (which means it's still an iMessage). However, the person you've been blocked by will never receive that message.
1) he could have his phone turned off;
2) battery might be dead;
3) he threw the phone away or got a new number;
4) he’s in jail and can’t use his phone.