Under the My Info tab on the left, you will see the people on your return listed. It is okay if your son is listed. Underneath his name, you will see Status. If it states Dependent, you have claimed him on your return and will need to click Edit to the right of his name and update. Be sure that you answer Yes to the question related to the other parent claiming him on the return.
If it states Nondependent - for EIC/dependent care only, you are not claiming him as a dependent but may still be claiming him for tax credits. Be sure to review the Deductions & Credits section of your return and answer the questions accordingly.