*I think this changes matters a bit, if your ex has cared for your son appropriately this often, including solo overnight care. As others have said, take legal advice, but you may struggle to justify restricting access if your ex was safely looking after your child, unsupervised, up until you separated.
Most alcoholics drink every day. If he has been abstaining when in sole charge of your DS while you were together, again, that suggests he may be able to do it (and he may be a problem/binge drinker rather than an alcoholic).*
This.
Is he actually an alcoholic, drinking daily?
Or is he a problematic binge drinker?
I’m presuming the latter, which is still horrible for you to deal with; but would support your decision to leave your son with him at various points.
If he is a binge drinker, when does he usually do it? If he works weekends, is it usually an end of the week thing, or earlier on?