Yes. Unfortunately, paying taxes in this situation is complicated. I lived and worked in Canada for 2 years, and my Employer hired Deloitte to file my taxes.
Even if my employer hadn't paid for the tax filing services, I would have personally paid for a professional tax advisor. Therefore, I recommend hiring a specialist In US and Canadian income taxes.
In particular, you should be concerned about a Canadian tax on his net worth, if he leaves Canada after living there for more than 5 years. Also you should be aware that American citizens are required to file income taxes in the US every year, regardless of where they live or work.