Unless I've misread the question, it's about CTC. EIP3 has nothing to do with this.

You mentioned that your client moved to Canada a few years ago and, by that, my presumption is that his place of abode is outside the US for more than half of 2021.  That would mean that §24(i)(1) doesn't apply to your client and CTC is, therefore, not fully refundable.

This also means that your client should not have received any advance CTC payments (assuming his 2019/20 returns show that he was living overseas) but I have seen the IRS sending payments to taxpayers who are clearly ineligible.

Edit: Just want to add that your client will still be eligible for CTC so long as all the other conditions are met.