The issue is CAN you be claimed as a dependent. Neither you nor your father gets to choose that. You either can or cannot be claimed as a matter of tax law. What he can do, however, is choose to not claim you. In any case, you are not entitled to the stimulus since he CAN claim you, even if he chooses not to do so. Since you already presented yourself as someone who cannot be claimed, it is possible (but we don’t know how the IRS computers check that) that if Dad claims you the IRS will reject his return. If that happens, he can either drop you from his return and refile his return or keep you as a dependent and mail it.
