score:59
Get a lawyer, get a lawyer NOW - you should indeed be worried about your son's next entry to the UK, not just the renewal in 2021 (which stands a high chance of being refused). You are going to need a lawyer experienced in both UK and Canadian immigration issues to sort this one out, this is far beyond what we deal with here.
To sum up from the comments some of the issues you need legal assistance with:
Holding a fraudulently obtained foreign passport may in-fact be illegal in Canada as well (I have an open Law question on this, so will update when I get a good answer), so theres potentially some issues there.