Shuttling between parents, boy acts out

It's possible that this is not about his mom. He might be working extra hard to ingratiate himself to you because he's afraid you'll drift away. He needs to know that you are always going to be right where he left you. Reassure him.

The way to help him make transitions is to give him the most predictable and calm environment and to encourage him to behave appropriately.

You and his mother should deal with this consistently. Make sure there isn't anything going on at her house to make him anxious. When he freaks out, calmly comfort and reassure him. Say, "Buddy, you're going to be fine. I've got to go but I'll see you tomorrow. Let's say a nice hello to Mommy and I'll see you later."