Custody

My boyfriends ex lives in Idaho and we live in NY. They have a 4 y/o son who they have joint custody with until he starts kindergarten, then they go back to court to decide where he’ll go for school. We have a 9 month old and he strongly believes a judge won’t want to separate siblings and only have them see each other on some holidays and in summer. His ex is a single mother and is in no way unfit she actually does a great job and is extremely hands on and has him on a schedule. He attends daycare in idaho but here in NY he doesn’t attend daycare because my boyfriend doesn’t think he needs to go and wants to have him home. My boyfriend met his ex in Idaho where he was living at the time then after they got pregnant decided to move back to his hometown. They lived here for a year then they decided to move back to her hometown. They ended up breaking up 2 months after that and he took their son and flew back to NY. She filed for emergency custody and the judge decided 50/50 and their son flies back and forth every 2 months. This has been the very costly arrangement for the last 3 years. My boyfriend is convinced he will be going to school in NY because his court appointed lawyer told him not to worry because his son is a NY resident and there would be no good reason to take him out of NY. But I’m just curious if that reason alone is enough for him to get full custody. She’s not unfit, she’s a good mom with a good job.. what are your opinions