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Amber Portwood isn't about to let her ex-BF hightail it outta Indiana with their son without a fight ... and says he has zero reason to leave their home state because he can do his job anywhere.

Amber filed her opposition to Andrew Glennon's proposed move to Malibu. As we reported, Andrew filed his own docs last month, informing the court and Amber that he wanted to with their 1-year-old son, James.

Andrew said he had nothing tying him to Indiana, but Amber says ... not so fast! In her docs, she points out her ex has put down roots in the Hoosier state -- he's lived there for 2 years and, obviously, had a child during that time.

As for his claim he works on left coast ... Amber says that's BS, because he's been a day trader -- which she points out can be done from anywhere. In the docs, Amber calls his attempt to move for employment "disingenuous."