There’s a long time rumor Fidel is his biological father.
IIRC, the Trudeau marriage was in early March, Margaret moved in with Pierre, and the press went bonkers stalking the young Margaret as the new FLOC married to an older Pierre.
Justin was born in late December.
Take a look at a Pierre yearbook photo, and an equivalent Fidel photo of about the same age.
The two share a lot of similar features, so yes, it's easy to make that jump.
I think that photo of Justin with Fidel was when Justin was two.