From a little experience in a similar situation, I would say that it
is not a matter of informing your manager about the situation, but
rather of asking him about it, keeping a couple of things in mind.
First: companies will sometimes hire someone as a potential
department head, but expect them to prove themself without much
support. Perhaps someone in the department was also a candidate for
the position, but the company didn't want to promote the person (yet),
waiting to see how an outside candidate would take charge. Maybe the
work you have been getting is something you should have delegated,
reserving for yourself the control and organization thereof.
Second: your manager most probably has an eye on the situation, maybe
an ear listening in the department. It is unlikely that you can
"inform" him of much, and not without sounding like you are
complaining about the organization of the company or of his and your
areas. ... Hmm, doesn't go down well.
If this strikes a bell, sounds like the situation, I would have
reservations about talking to him, unless I had a strong plan to
present. That could include explainging that you wanted first to
prove your expertise in the nitty-gritty of the work you have been
doing (hopefully better than that of the others) to win his and their
respect, with the goal now of being able to help and improve the
group's effort and output, supported with some organizational
suggestions, which are improvements.
If you are seriously thinking of looking for a new job, and the market
is pretty positive - and the above does strike a bell, i.e., your
manager could be testing you, and if the situation is "uncomfortable"
for you, like maybe he also would prefer that you seek a different
job, I would consider being open with him. He made a mistake, either
in hiring you or in his management of your integration (he has a boss,
too). If you think that that is the way it is, he would prefer that
you leave voluntarily, which looks better for him and for you, so he
could be agreeable to mutually acceptable solution that allowed you
time to seek a new position while he could handle the situation in the
department with less waves.
If I am way off target - this is just a free comment, no "answer" to
your question - just ignore my remarks. They are well-meant but
could be off-track.