Yes, your son can definitely go abroad for higher studies with two marksheets, one from CBSE and the other from NIOS.
NIOS (National Institute of Open Schooling) is recognized as equivalent to other national boards like CBSE. This means your son's NIOS chemistry marksheet will be accepted by universities worldwide. Universities abroad typically verify academic credentials. They will assess both marksheets to ensure the student meets their admission requirements. Each university has its own rules about accepting multiple marksheets. Some might require additional documents or explanations.
While having two marksheets might require extra paperwork, it generally won't hinder your son's chances of studying abroad.

I hope this information helps.