This question is far too complicated to be answered in this general online Q&A forum. Your son, or you, or both, should bring all your facts and figures to an estate planning attorney who is also familiar with international estates.
There is generally no tax on a person receiving an inheritance. But because your son is a US green card holder he will likely be subject to the US estate tax system. As a Canadian citizen he does not get the same benefits of our estate tax exemptions. I suggest he discuss his situation to determine his estate tax exposure. Often times in a situation like this a trust is set up in the foreign jurisdiction for the benefit of the US residence - but non-US citizen to minimize the US estate tax exposure.
The above statements are provided as general information and not intended as legal advice. Each matter has its own set of unique circumstances that cannot be adequately addressed without consultation. You are strongly advised to hire an attorney licensed to practice law in your state to represent you.
It sounds like both you and he have the means to hire an attorney to discuss estate planning issues and concerns, This forum is for those needing legal answers to simple questions. We volunteer and donate our time to help fill the void in legal services to those in need.
IF YOU FOUND THIS ANSWER "Helpful" or " The Best Answer" YOU CAN THANK ATTORNEY RADDATZ BY MARKING IT SO because Avvo awards the attorney points. MS. RADDATZ is donating her time and talent by answering questions to help those in need of legal information. This is NOT a consultation and in no way creates an attorney-client relationship. YOU SHOULD ALWAYS PERSONALLY CONSULT WITH AN ATTORNEY IN YOUR LOCAL AREA who has specific expertise in the area of law you are asking about.