Barry must have x has been relieved when he found

Hi guys, I have a simple question but that’s is annoying me too much.
I learned to use has been to he she and it, and have been to you, we, they.. But and when there is a hidden subject, what can I do?
For example..
Barry must have been relieved when he found his missing cellphone.
Barry for me can be replaced by he, so would be has been instead of have been in the phrase above.
The question is, why it’s have been instead of has been, specifically in this case.
Thanks for the help, have a nice day!!