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So in short the only way to go to canada is using RTD and apply for TRV to visit relatives in canada?

He will not receive a TRV given that he is a refugee who lives on PNG. Getting any form of temporary visa to enter Canada will likely be refused due to overstay risk. If he wants to move to Canada he needs to look at various economic immigration pathways.
 
So in short the only way to go to canada is using RTD and apply for TRV to visit relatives in canada?

Agree with above. Zero chance of a TRV. If he wants to see his relatives, they will need to visit him.