My Girlfriend's son loves grandparents more than her??!?

I have been living with my GF over a yr now, we got a house together and are doing fine. her 4 yr old son however has some issues when it comes to his grandparents..i dont really know how to react cuz i never grew up with grands, but he mentions his grandma in nearly every sentence that comes out of his mouth throughout the day and night. and when we are done with bedtime battles he sometimes refers to her as well. but every time it is time to leave grandparents house he throws a major fit, cryng hysterically and wont say goodbye to anyone, then continues the fit the whole way home. EVERYTIME we give him pre emptive warnings about his atitude there and when leaving, and he still keeps up the atitude and manipulates his way because i cant really get too strict with him in front of them, yet he only listens when i speak to him really. our house is nice..nicer than grandma's and he never wants to come home and makes his mom upset all the time. it seems he loves grandma more than mom????