If he was filing as a permanent resident of Canada for those 3 years, and he will now be a permanent resident of the US, then you mark the date of departure, and he is taxed in Canada only on income earned up to the departure date (regardless of where it is earned).  You report the income earned in the US after departure just for purposes of prorating the non-refundable tax credits.  

He has to file a US tax return if he is now qualifies as a permanent resident of the US, and will file a US tax return reporting income earned from the date of immigration.  

Everyone has to have one country where they are taxed, basically.  They can't just float around homeless (in other words, the courts have ruled they cannot be "stateless").  The tie-breaker rules in the income tax acts will look at various factors to determine where they are considered resident, and the tax conventions between the countries also will have tie-breaker rules.  If your client has been considered a permanent resident of Canada, he only stops being taxed as a permanent resident when he takes up permanent residence elsewhere.  If you look at form NR73 Determination of Residency Status available on CRA's website you will get an idea of the type of questions he would have to answer if CRA disputes that he is an emigrant.