Han finds out Snoke has been trying to lure his son to the dark side. Angry that Leia was aware and never told him what was happening, he impulsively decides to take Ben and disappear. I'm picturing this happening when Ben is quite young (maybe 5 or 6?) and just want lots of protective dad Han cuddling his little boy and promising him he'll protect him from the bad man who whispers terrible things to him and wants to take him away. If Uncle Chewie comes along for the ride all the better.