OK, first of all. The visa holders do not have to travel by September 30th. Depending on medicals, they will have up to 6 months after the visa is issued to enter the USA for the first time. So for instance, let us imagine you have your interview in June, you would have until that date in December to travel.
Next. You should list your son whether you intend to get him a visa or not. It is critical that you tell the whole truth on the paperwork. So - if you have a son, list him. The decision of whether he goes through the visa process or not, and then whether travels with you or not is incidental. However, if you want to get him a visa, be prepared for some questions about your legal custody of the child and the mothers role. If you do not marry her she could not benefit from your selection - so they will be sure to ask about her intentions and rights.