There might be many reasons based on what more information you provide for this but I am guessing the most likely scenario - 

There might be a chance that once you input his (your son) details in your software then it added him as your dependent. You check your draft 1040 and see if that is the case. If so then you delete his information from your tax filing and this should resolve the issue. Your software is probably asking this details because it sees you using him for the previous years (assuming you are using the same software that you used for the previous years)