i need help....like bad?

okay my boyfriend lives in new mexico && I'm in texas but he was supposed to move back this summer and well he did but then his step dad..{whom he was supposed to live with} got a job in Dallas and was moving at the end of the summer and my bf didnt want to move to Dallas so he jsut went back to new mexico. But come to find out since his sister stayed here his dad is letting her stay here and finish the semester and buy the time its over he will be almost 18..

my question is...he wants to come back now and his mom is very mean {im not eagerating either} but i cant think of wat to to tell him to tell her bc she has custody over him his dad dont..but i need help figureing out wat to say to her..any thoughts??