I am in the same situation like you. For my case I am sponsoring my husband and dependent child born outside Canada. After landing as a PR, I and my elder daughter returned from Canada within one month like you. As we were living together all the time except one month, I couldn't prpovide any kind of communication. But I showed my joint account, Proof of living in the same address, photos of different occasions with all of our family members. I also included photos of my child's birth and photos of different event . So that they can understand he is my son. For your case as you are sponsoring your son, it's important to provide proof that the younger one is yours. Show lot's of photos, cards, pregnency reports, etc etc. Otherwise they may ask for DNA test if they are in a doubt.