http://www.inspection.gc.ca/english/.../canin2e.shtml
You had a nice Border Services Officer. I would also check with the appropriate federal dept. in the USA. My sister bred, showed and judged Belgian Sheppards for years both in Canada and the States. There are requirements for Canadians bringing pets into the USA similar to ours here. Given it is a young dog they may require proof it was whelped in the USA.