My son seems to talk more and more clearly when he is visiting his grandparents and father why?

I dont know why but my 2 year old seems to do more talking and interacting when he is with his grandparents and father. I know they spend their time(every other weekend) completely focused on him and doting on him and that is wonderful to me but i am afraid that these weekends make him feel neglected when he comes back and has to share the spotlight with his brother who is 10 months old. I dont know what to do or how to get him to communicate with me the same way. It makes me feel very insecure as a mother. I am doing the best i can. Also maybe i should mention that his grandmother has a degree in early childhood development and slpecializes in getting children to communicate mostly developmentally disabled which is not the case with my son. He is very intelligent and happy and secure at home so i cannot understand why this is happening, please help.