Your state publishes the child support payment guidelines, look them up. He probably did the math with 50/50 then the every other weekend thing. I'm betting his motives are financial.
Yeah, most of the time, they say they want 50/50 so they won't be responsible for child support. Child support is typically figured by a formula that is based partially on the number of overnights spent at each parents' house.
Chances are high that if you agreed to joint custody,with you as the physical guardian, he will lose interest in a while. It'll cut into his drinking time, having the kids around for an entire weekend.
IME, a guy with the narcissist/sociopath tendencies may be motivated to exercise visitation time in order to punish you, groom the kids, and keep up a defensible "good guy" cover. This is how it was with mine. If he was just an A, he'd probably fade out of their lives, but the narcissist/sociopath stuff throws a wrench in it. Bank on him being unpredictable, is my feeling, which is all the more reason that the kids should be with you.