User blog comment:NickHunter26/Where was Jesse Turner ?/@comment-1975828-20160521090538

Sam and Dean clearly didn't want Jesse to fight in any war, and preferred Jesse to be a normal child. When Lucifer was re-sealed, I doubt he had any of his powers left, and by the time Lucifer returned, he would be stuck with a huge amount of power he had no time to master.

Also, according to Lucifer, he's been gone for five years Earth-time. Jesse would be a teenager, and I'm pretty certain he'd be harder to handle at that stage. Another thing, I doubt Sam and Dean could figure out where the boy is.

As for the Leviathans, they're virtually indistructable, and all they do is eat, eat, eat. Eve only cares about the safety of her children and the Natural Order, so what is to say these two will agree to help, let alone work with each other?