One bag of layer pellets (lasting a month or two), when the rooster is already an adult, should not kill him and will probably not even cause problems that you can notice. Some people have roosters that eat layer pellets for their entire adult life and appear to remain healthy.
But of course there are no guarantees, and some other people with roosters DO see problems if the rooster eats layer pellets for an extended length of time.
In general, extra calcium is more likely to cause trouble with young chicks rather than mature chickens, and it is more likely to cause trouble if the layer pellets are the only food available (so a rooster that ranges freely might fare better than a confined rooster, because he has more other things available to eat.) The calcium damage also tends to be cumulative, so it builds up over time rather than appearing immediately.
If you tend to worry about your chickens, you might be happier if you just exchange the feed. I don't know how much trouble that might be for you, or how much you worry about the health of each individual chicken, but both of those will affect what is "best" for you to do.