Yes, you are right. Your husband can only apply for a visitor's visa to Canada if he has lawful status in the country he is applying from, in his case the US and then even if he has lawful status, there is no guarantee that he will be granted visitor visa.
http://www.canadainternational.gc.ca/buffalo/imm/visa_temp.aspx?lang=eng#REQUIREMENTSALLYou must provide proof of lawful temporary status in the United States