Seeking Modification of Custody When the Child Doesn't Live With the Custodial Parent

My question involves a child custody case from the State of: California/ Washington State

I was a single mom, I had cared for my son since the day he was born up until he was almost 8 years old. I had started a new job and relocated to a different city and wasn't able to get child care for my son so I allowed my son to go stay with his dad. We made an agreement that my son will stay with his dad for 1 year only. After the one year was up my ex wouldn't allowed my son to move back with me. I didn't wanna fight with my ex (we all know it's stressful and frustrating when it comes to child custody agreements), so I decided to let my son stay one more year. (It's a total of 2yrs now)
I have now moved out of California to Washington state and i want custody of my son back. My ex does not take care of my son, my son lives with his grandma ( my ex mom ), my ex does not work, he lives with his gf, he only picks up my son from school or has his gf picks my son up, and sometimes leaves my son at his gf house for hours while he's gone somewhere else. Basically my son is being raised by his grandma like he has no parents. I work and have a stable home with my Fiance, my son is turning 10 soon and is asking to come back to live with me, but his dad is not allowing it.

My question is will I be able to get my son back? Can I get full custody and move my son to Washington with me? At least I know my son will be in the same home with me and not leave him for someone else to support and care for when his mother can care for him.

You're not clear on one thing - does Dad actually live with Grandma too, or does he live somewhere else?

Dad lives with his gf and her four kids, in a different home.

Okay, thank you for clarifying.

You can file for custody based on your child not living with his other parent. This would be the change of circumstance you need. Do you think Dad is going to object?

Yes, I'm 90% sure dad will object.
My ex has always been a liar and I know he will lie to judge/court that he lives in the same home as my son and will have his mom cover for him and my only proof is my son telling me this. My ex will say whatever he can to keep my son from me, cause he's afraid he's going to have to pay child support.

Whats the chances of me wining custody?

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Quoting Mk733

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Yes, I'm 90% sure dad will object.
My ex has always been a liar and I know he will lie to judge/court that he lives in the same home as my son and will have his mom cover for him and my only proof is my son telling me this. My ex will say whatever he can to keep my son from me, cause he's afraid he's going to have to pay child support.

Whats the chances of me wining custody?

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What are the chances of you proving where your son lives? What address is on the school records? How involved have you been with your son?

I moved to Washington state last year 2015 of may.
I call my son every week or every other day, and I visit during the holidays.
I'm sure documents from my son's school has his address on there, but my son lives with his grandma.I don't understand why I need proof of my son's address?

Because you have to prove there has been a significant change in circumstances. Your ex allowing your son to live in a different home than his is what you need to prove. Talk to the neighbors, do what you can to prove the situation is as you say.

Are there ANY court orders indicating custody?

My son's supposed to live in the same home as his grandma (my ex's mom) and his father, but only y son lives with his grandma and my ex lives in a different home. Which my ex is supposed to be living in the same home to care for my son.

I was the primarily parent before. Then we had an stipulation agreement, that my son will live with his dad till the end of school year of 2015, but after end of school year in 2015, my ex did not allow my son to move back with me. The agreement we made was documented, notarized and signed by judge.