Help child to join mother

[Ajayv]

Child to join mother
Help confused....

My partner is here in london with me on a unmarried partner/spouse visa since September 2017. Her permit expires in May 2019 and then we will apply for an extension.
She has a son from her previous marriage who is living with his aunt (partners sister) in mexico. We want the son to join his mother here in london as soon as possible. The questions are :
1. Can I apply for the son to join his mother before she has extended her current permit.
2. Can the son travel here as tourist and then I make an application for him to remain.
3. Do I need to apply separately for him and his mother to remain permanently or can I add him to his mother's extension application.
4. If I add him to his mother's application then do I need to pay for both....or do mother and child count as one application.

I would appreciate any help I can get and just to clarify I meet all the financial requirements and from my understanding he does not need English certificate as a child, correct me if I am wrong

Thanks

[CR001]

Where is the child's father??

How old is the child?

[Ajayv]

The father is in Mexico. And the child is 13 years old

[CR001]

Does your partner have sole responsibility for the child and official evidence of this?

[Ajayv]

Thanks for your attention.

My partner does not have sole custody of the child but has joint custody with the child's father. The child has not lived with the father since 2013.
The father has no objection to allow the child to travel as he has little interest in the child's future. Even though it's joint custody the father has not contributed towards the child in anyway since 2013.

[Ajayv]

Very silent on my topic......is it complicated

[CR001]

She would need to prove sole responsibility. See link below.

immigration-for-family-members/sole-res ... 13087.html