As far as the law goes, yes, you can lose your job when he comes back to work; not because there is a law I can link you to that gives the employer permission to let you go, but because no law exists which says that he can't. Google, at will employment.
Therefore, this becomes a matter of your union contract. If the contract allows you to be let go in this situation then you can; if it does not allow it then you can't. You being the only one here who has read your CBA, that's as far as we can take it.

