He seems to be very upset at its loss. His sister's arrival is also an important event in his life. I would recommend asking your son if he would like his pacifier back. If he says yes, then the rule could be that he uses it only at night in his bed. More important however, I think that you need to help your son express his negative feelings towards the coming of the baby.
All children have negative as well as positive feelings towards a new sibling.
They need to be able to express them in order to find out that it's okay to have them. Talk to your son about how difficult it must be to share mom and dad's attention with a baby. The better your son feels about his own feelings the easier it will be for him to accept growing up and eventually giving up the pacifier. You should not rely on this information as a substitute for personal medical attention, diagnosis or hands-on treatment. If you are concerned about your health or that of a child's, please consult your family's health care provider immediately and do not wait for a response from our professionals.