If he is shown on the birth certificate as the father, my guess is that there is very little he would need to show in addition to that. There probably are some things (passport renewal?) but I don't know, I'm not an expert. A lawyer is likely needed.Sir/Ma'am may i also ask if is there any things that my ex might do for my son in canada that he needs to show proof that he has the full custody of a child? Like maybe for traveling in the future, sending him to school etc.
Unfortunately the most effective step is the police. I'd say you have to push forward: a parent who does not have legal custody who is not returning the child when and as agreed is very much at fault.
But you will have to go to the authorities and there may be additional required steps, like formally writing and delivering a demand notice (in short, a letter saying return the child now).