I keep re-reading your post and thinking maybe he just needs time to himself after spending all day with you and your son. Please don't take that wrong. If he's an introvert like I am, it's tiring to spend a lot of time with people. We need time to 're-charge.. and even the constant messaging interrupts that. I also get worn out by my gf's 8 year old son who talks incessantly.
I say all of this because his drinking seems more like occasional binge drinking to relieve stress, than alcoholism, especially when he drinks so sporadically.