Searching for Emails assigned with Categories

My boss has marked some emails with categories. I have full permission to
his inbox and sub folders and have added his mailbox to my outlook.

However when I search, selecting a specific category(ies) I can only do this
on a folder by folder basis.

Is there any way I can do a global search of the category(ies) I want
without having to select each folder individually run the search then move on
to the next folder.

I know you have a search folders section which can display categories for
yourself but even with full permission to my boss's outlook inbox I don't
seem to have this facility.

I am not sure whether this makes sense but any responses, suggestions would
be greatly appreciated.

Many thanks

Heather