It sounds like they may be more stringent about this in USA and Canada? Everyone I know does this if they will be travelling wihout one spoouse or the other. It is not worth the hassle if they ask and you don't have the letter, and sadly, it is a policy that reflects the fact that the vast majority of kidnappings are committed by a parent.
I understand your worries but I'm sure a passport will suffice. And just for reassurance, maybe take his birth cert. Surrly it is very unreasonable to expect that a child can only leave the country with both parents present?? Well I think so anyway!
Let us know how u get on, would be useful for a lot of us to know x
Seems like you do need permission. Strange I have never known of any of my friends or family who have travelled with their child to have needed it