@Bs65 thanks for the reply. On the COPR it is clearly mentioned that this document is not for travel , and his visa will expire after few days upon his arrival.
Correct. What that means is that he cannot travel using the CoPR alone, he must also have a valid passport. Assuming as you say, he will declare PR and 'activate' his CoPR on arrival in Canada, at that point as long as he can provide CIC with a Canadian residential address, they will then send the papers on to the agency which issues PR card. Unfortunately, these take several weeks (maybe 3 months) to arrive. But at the point of declaring PR at the airport, he is then a permanent resident of Canada.
@Bs65Wil he receive and travel document at canafian airport upon his arrival which he can use to travrl back after 3 weeks ?
No he will not.
He can leave Canada easily. His problem will be that as a Canadian permanent resident, no airline will allow him to board a return flight to Canada without his passport (of course) and either a valid PR card or a Permanent Residents Travel Document (PRTD). PRTD's can only be applied for outside of Canada at the local Canadian visa office for the country he will be in at the time. There are several documents he will have to provide and of course another form. Issue of a PRTD timescales vary by visa office but can be from a few days to a few weeks to process. His only alternative would be to fly back to the USA and cross into Canada via a land border - CBSA can then check on their systems at the border to confirm he is a PR (airlines have no such access to these systems).