4. If you are living and working in Canada now and move in May 2013 to the US, you would file taxes both in Canada and the US for the year of 2013. They have a tax treaty so you will not pay double tax.
Depending on your combined US and Canadian income you might still end up paying substantial tax in Canada. Both, US green card and Canadian PR has to file their taxes on global income and not only on the income earned in US or Canada.
4. After that, you may ask Canada to consider you a non-resident for tax purposes as you do not live there any more. When you move back to Canada, you would start filing taxes again but as a US PR, not filing taxes in the US can cause them to feel that you have abandoned your PR.
If he does not file Canadian tax for the years he did not live in Canada will that be an issue when filing for Canadian citizenship? My wife got Citizenship Query and the lady who interviewed her before her citizenship test sarcastically told her that "oh you are paying taxes in Canada? Great! Here is the query and bring your tax documents for past so and so years" She was directly implying that since my wife also has US green card she was not filing taxes in Canada.
4. In order to keep a US PR, you can not stay outside the US for more than a year at a time but 6 months is better.
You can keep your US green card by filing returning permit if you are out of US for more than one year. However since you plan to get Canadian citizenship, in all probability you will get query - also note that in Canadian citizenship application they do have a question "Have you obtain permanent residency in another country?" There is also common misunderstanding that you cannot stay out of US for six months straight - the rule is now changed to cumulative 6 months in a year instead of continuous 6 months in the past. Also keep in mind never to lie to Canadian or US border agent about your entry and exit. Within next few months both US and Canadian immigration will be using a single computerized system.
Of course this information is like a-friend-of-a-friend-of-a-friend told me but it is better to be safe than sorry.
Novagui - Please note that when you enter US as green card you can bring all your possessions including your car duty free - you have one year time from the date of entry to complete that transaction. This rule is similar to Canadian PR rule.