In fact I would go as so far as to say that he has just been royally screwed over. I've been in this position with an ex-college of mine. She was the Assistant to the Manager of ........... Uk Ltd. He resigned. She was told that she was to act as manager until such time as a replacement was appointed. Despite being the de facto deputy manager, she was not going to get the job. She asked my advise. I told her to refuse, until or unless threatened with dismissal. Then do the job under protest. I also told her that she should secure other employment. Then resign, making it clear in her resignation that she resigns because of the companys actions.

My reasoning is/was that:

1) If your good enough to to the job "temporarily" then you are good enough to promote.

2) Doing the job and then having a new manager brought in over you will be cause anyone examining your work record to conclude that you were not up to the job. In consequence you were demoted for cause .

3) The manager is actually expecting you to show Loyalty and Commitment to them. Whilst being grossly disloyal to you as their employee.

Ok, advice. Approach your Manager. Ask him for the reasons why, despite you being expected to do the job "temporarily", you are not being considered for or being allowed to apply for the position. Do it in writing. Keep a copy.

What he does is entirly down to him. Personally I would not. For the reasons I have given.

By all means do the job for the experience but be aware that:
1) You can never go back to being one of the boys.
2) You will always be the spare, not the heir.