From what I understand, you need a status refusal to get the TRP because it's reserved for people that are normally inadmissible to Canada. In order to apply for one before coming to Canada, you would need to apply for a TRV and get denied. Since you're visa exempt, there's fairly good chance CIC would approve a TRV for your son with proof you plan to move back to Canada, etc.
However, you don't *need* a TRV. This is one of those situations where it will boil down to what the IO decides when the decision is in front of him/her.
Personally, I would travel to Canada and ask as I enter. If that doesn't work, I would apply for an extension through MyCIC online/flagpole at the border and try for a TRP again after a few months. You may have to ask several times. Many IOs will see it as no problem your son is in Canada because you're a PR and he's visa exempt. They won't see the need for a TRP or TRV. That's why you should be prepared to explain why you want one.
However, you don't *need* a TRV. This is one of those situations where it will boil down to what the IO decides when the decision is in front of him/her.
Personally, I would travel to Canada and ask as I enter. If that doesn't work, I would apply for an extension through MyCIC online/flagpole at the border and try for a TRP again after a few months. You may have to ask several times. Many IOs will see it as no problem your son is in Canada because you're a PR and he's visa exempt. They won't see the need for a TRP or TRV. That's why you should be prepared to explain why you want one.
