My daughter who is dual US and Canada citizen lives in Toronto. She will be coming to visit in the US with her boyfriend who is a Canadian citizen with Iranian birth. Can he be denied entry to the US?
Yes, he can be denied entry to the US, just as every Canadian citizen (who is not also a US citizen) can. However, if you're asking about the travel ban that applies to citizens of certain countries, including Iran, the answer is that your daughter's boyfriend does not fall under the ban if he travels with his Canadian passport. The relevant text is found in presidential proclamation 9645 at section 3(b)(iv):
(b) Exceptions. The suspension of entry pursuant to section 2 of this proclamation shall not apply to:
(iv) any dual national of a country designated under section 2 of this proclamation when the individual is traveling on a passport issued by a non-designated country;