I need some advice on my S/O & alcohol

Okay. So here's some background. My fiancé were once in a rough place... we were only dating and Our relationship was horrible... my boyfriend (now fiancé) would drink whole fifths while apart  and treat me like shit. Break up with me in the middle of the night, all that. We ended up back together eventually. Things were 100% different.. so a few months ago, I went away to basic training and my fiancé was super low & sad. At least twice while gone he drank a whole fifth of whatever alcohol to the point where he passed out and was completely incoherent. So now, anytime there's any alcohol in the house and wants to have a few "shots" which is actually him turning up the bottle for a few Chugs, I get upset. I don't know if I'm out of line or if I'm being naggy. To me, it'd be okay if it were every few days or so, or farther between, but it's every night, like he "has" to or feels the dire need to drink some sort of alcohol. Especially every weekend, he has the dying desire to drink. I just get worried. His father who passed away from a heart attack, had major alcoholism... when I express this worry He gets defensive & claims  "I don't drink that much, I don't have a drinking problem." I just don't know how I should feel or what I should feel... never in my right mind would I ever think that drinking a whole fifth of anything ever would be healthy or stable... can anyone help??