rzchen

Then yes, if you have a reason that comes under one of the criteria ('medical or humanitarian emergency') you can apply for an expedite (with proof of your reason i.e. medical records & doctors letter). 
 
Edit: if you could fill in your timeline too that would be helpful. It's hard to advise without knowing which visa category you're talking about, if your PD is current, etc. 

My case was expedited only for getting the IL sooner.  Justification was national interest provided by my company.

Tax transcripts also work. They may not ask for it if you have already uploaded latest tax transcript to NVC. It appeared that they already have all info about IRS taxes in their system. They usually ask for W-2.

See this list: https://travel.state.gov/content/travel/en/us-visas/Supplements/Supplements_by_Post/MTL-Montreal.html

https://www.fickeymartinezlaw.com/immigration/consular-processing/canada-police-certificate-us-visa-document-requirement
 
https://travel.state.gov/content/travel/en/us-visas/Visa-Reciprocity-and-Civil-Documents-by-Country/Canada.html
 

This is the one you need. The reason MUST say other. 
 
If you have any other police certificate you will get your visa refused and you will have to re submit this one! 

Yes
 https://travel.state.gov/content/travel/en/us-visas/Supplements/Supplements_by_Post/MTL-Montreal.html

If you've applied outside US so your case is going through IVP. So, you must take the medical examination with the authorised doctors who suggested by US Consulate. 

If you are doing it through AOS you can use a doctor in the US. If you are going through consular processing it has to be from outside US

You will need to bring your Interview Letter to the medical exam appointment. So, no. 

In this context. It is administrative processing. It happens if you are not approved for the visa after the interview in Montreal. There are many reasons. Sometimes you are missing documents, or something comes up during the security checks. 
 
Currently the wait if you are put into AP is up to 18 months after the initial interview and montreal have been putting a large number of people into this category. There is a whole other blog post for that because it happens to so many people. Sometimes after this process people are approved. Sometimes rejected and it needs to be appealed - which is also a very long process. 

Your son should know. Ask him to check his visa and all the other paperwork related to the J1 visa. 

He will know, it will be in his Passport

No

As long as he complied with all requirements of previous visas, then many many beneficiaries of immigrant visas have previous US visa histories. Compliance and no unauthorized activities or overstay are key.