What the kids call a step-parent is so trivial in the world of crap these kids are dealing with that it is not worth spending one second worrying about.
Their father is an alcoholic, too. And "tapering down" to one beer is not going to keep him sober. The ONLY virtue of tapering down is when one is drinking so much that not tapering would lead to physical withdrawal symptoms.
I would not marry this man until he has at LEAST a year of solid sobriety. Preferably in AA, though that isn't essential for EVERY problem drinker. But during that year you should see personal growth and maturity and increased responsibility.