The only way he can even enter Canada as a visitor is if he is granted a Minister's Permit.
The chance of obtaining a Minister's Permit is dependent on the date of the conviction (19 years is fair), what kind of conviction it was (a violent conviction of any sort is practically an automatic rejection), what sentence was given and served and the reason for entering Canada. It can take 6 months to get this permit.
However, the likelihood of him ever being able to obtain citizenship in Canada is virtually nil. If you want a life with this man, you're going to have to move to the US. He can't move here.
Canadians on here would be able to answer this question correctly for you. From a foreigners perspective, well I'm from Australia, I would think after 10 years a person's conviction would be wipe depending on how serious the crime (felon) is. If the crime (felon) is severe, for example murder, ra-pe, fraud, etc, I highly double the Canada's immigration department would welcome the person into their country with open arms, let alone grant a visiting visa.
I would certainly hope so, 19 years should be plenty of time for his past to be behind him. However I'm not familiar with Canadian law. I suggest asking a Canadian police officer or attourney to eliminate the guess work.