Visa

Regarding his previous B1/B2 travel history to the U.S., there shouldn't be an issue with further travel while his case is in administrative processing. Although there's a slight possibility of being turned away at the airport, Customs and Border Protection usually treats spouses of U.S. citizens with consideration.

To avoid potential travel hassles, your son-in-law can clear U.S. immigration at Dublin Airport, which offers a more convenient option.

A Canadian immigrant doesn't enjoy any special advantages over others. However, once you attain Canadian citizenship, numerous new opportunities become available. For instance, you become eligible for Treaty Trader and Treaty Investor visas like E-1 and E-2, as well as TN visas. Additionally, for most visa categories, you don't actually require a visa stamp. As a Canadian citizen, you can enter the United States without a visa, except for E visas and K visas. Visas such as F-1 and tourist visas typically aren't necessary; you can simply arrive at the border or airport with your Canadian passport. That's how the process works. It's important to note that being a permanent resident of Canada doesn't provide any advantage in the United States.

The processing time for the Canadian passport might be longer, so I suggest you check with the consulate. This situation appears unusual to me. In my understanding or recommendation, the law implies that once you take the oath of naturalization, it's the same as in the USA. I'm unsure about Canada's specific rules, but when you take the naturalization oath, you typically cease to be a citizen of any country except Canada. 

You may need to explore options to expedite your passport application, perhaps by paying emergency fees or any other required charges. I believe you should be able to enter the United States with a Canadian passport and an I-20 without needing a visa. Canadian citizens usually do not require a visa for entry.

Whenever the authorities, whether it is a judge or the government says "canceled without prejudice" that means they are canceling for some technical reason. This is not going to cause any harm if you want to apply for the same visa again. This therefore means cancellation without affecting your ability to apply again. It is not a black mark against you. 
 

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Note: Unless the context shows otherwise, all answers here were provided by Rajiv and were compiled and reported by our editorial team from comments, blog and community calls on immigration.com. Where transcribed from audio/video, a verbatim transcript is provided. Therefore, it may not conform to the written grammatical or syntactical form.

Note: Where transcribed from audio/video, this is a verbatim transcript of the referenced audio/video media delivered as oral communication, and, therefore, may not conform to written grammatical or syntactical form.

Note: Where transcribed from audio/video, this is a verbatim transcript of the referenced audio/video media delivered as oral communication, and, therefore, may not conform to written grammatical or syntactical form.