I'm not familiar with pro-mils. However if it's the equivalent of BAC 0.8 or higher - he's currently inadmissible to Canada (meaning he can't enter the country due to his criminal conviction). I suspect this is why the processing of his ETA is taking a while. I would not recommend that he fly to Canada until the outcome of the ETA is known because if he's inadmissible - he will be sent right back home on the next flight.
If he is in fact inadmissible, he will have to wait until five years have passed since his license suspension ended. Once the five years have passed, he can apply for rehabilitation. If the rehabilitation application is approved, he will no longer be inadmissibile to Canada.
So it's quite possible you're 3+ years away from him being able to enter Canada and visit.
Again, you should wait to see what the results of the ETA are before putting him on a plane.