A child of nine ought to be able to cope with you being away for a month or so; so are you sure the bed-wetting is only caused by the thought of not being with you and his granddad?
I would have a word with his school teachers; explain about your journey and ask whether there is anything going on at school that could be upsetting him.
Unhappily, even the best teachers don't always know if there is bullying or unpleasant teasing going on, but by asking the question, you may be pointing them in the right direction, even if they answer your original query with a resounding no.
Next on my list would be whether your grandson has started drinking too many fluids just before bedtime? Or started sleeping more soundly, so he doesn't wake when his bladder says it needs relief? As a child I vividly remember dreaming I had woken and gone to the toilet only to wake up in a wet bed. Perhaps the same thing is happening to him?
If the answer to these three questions is no, then perhaps a word with your family doctor is indicated. Boys and men do get chills on their bladders or kidneys too, even if we do tend to regard it as a female complaint!
Is your GS peeing more during the day than before, or forgetting to go, so that the bed-wetting is part of a change in his habits?
I agree that bed-wetting does tend to be a sign of stress or worry, but there could be a physical reason.
Please try not to be too hurt by those who have jumped down your throat because you and your husband are going away, on what I am sure, is a well-deserved trip.
One final thought, make a calendar, either on paper or on the computer for your grand-son with the date you are coming home written in in LARGE LETTERS, so he can cross off all the days between when you leave and when you get back at bedtime every day.