J.J.
We are in the same situation with a new baby due next month. My son doesn't seem to care so much about the monitor, but since the kids bedrooms are upstairs and our master is down, I feel more comfortable keeping one in his room for safety sake. If he had a problem, I wouldn't be able to hear him and he wouldn't be able to get downstairs to our room. I say if your son wants to keep it, he should. The transition will be hard enough on him anyway, let him keep his "radio."
I was actually thinking about posting a similar question to yours. I have seen dual monitors for two kids rooms and am wondering if we need to get one of those or just buy another one for the baby. Having two not made to go together could cause some feedback or other interference. I am very interested to hear what others have to say.