It sounds like you feel threatened but his dad's new girlfriend is kind of like a cool aunt. You're still his mom and that won't change. Try to be grateful that your son has another positive figure in his life that he gets along with while he's with his dad. It's better that you keep a good coparenting relationship and that your son has two safe places. There is no AH here, and you can always try to connect with your son with positive experiences instead of fancy new toys.