There is no limit as to how many times he can visit the US as a visa exempt Canadian tourist. He should be mindful of the time though and not spend an aggregate of more than 6 months in the US in any given year. There is no prohibition to you being married and living in different countries.
A DUI offense should not cause any problems for him visiting as long as he didn't kill anybody and has handled everything attendant to the arrest (i.e. paid court costs, completed probation, etc.). As a visitor, it likely would not even arise in his border crossing unless he starts overstaying his 6 month stays. Even so, DUI without vehicular manslaughter is not an inadmissible offense.
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