6 year old is super jealous of sibilings 😰 help!!!

Here’s a run down of the situation. I have been a single mom for most of mine and my sons lives...it’s always been me and him living alone. He’s my baby boy. Ya know? So fast forward a year I am now engaged and my fiancé has two little girls. One my sons age 6 and a 3yo. The tough part is we only get the girls every other weekend...so when they are here we usually aren’t as tough on them. We always are fair between the kids and always make sure they all feel included. We also have one on the way my son is SUPER excited about having a brother....But My son is suffering from only child’s syndrome sooooooo bad! Every time they come he regresses so bad! He throws fits and acts extreamly jealousand impatient!! I hate it for him I really do. I mean he will adjust eventually right? Does anyone have suggestions??