Neat trick, considering the Canadian Constitution wasn't written until 98 years after he died. If you're thinking of the British North America Act of 1867 then Benjamin had nothing to do with that. At the time he was merely a London barrister and not associated with the government.
It's an interesting story.
Search is you friend. Benjamin was never "merely a London barrister". He was a famous barrister, a QC, and the author of a legal textbook which is still in use today. He represented Canada in several important cases, then became a key adviser both to the privy council and to the judicial committee on the House of Lords. Benjamin argued that the Canadian central government was formed only for the purpose of protecting Canada from the United States, and that all other powers rightfully resided in the provinces. To this day the Canadian government is more of a true federation than our government and this is largely the legacy Benjamin's legacy.