Minor child of new British Citizen

[Boasleyboy]

Thanks to the excellent advice received here, my partner became a British Citizen by descent in February this year (2020).
Her son is 17 and living with her brother in the US, due to his father's homelessness and addictions.
She would like to bring him to the UK to join us permanently.
Could anyone advise me on the best route to get him a permanent/long-term move to the UK?
Thank you in advance.
Regards
Mark

[secret.simon]

She'll need to pull her socks up and get him to the UK before his 18th birthday, as the odds of his being able to move to the UK will lengthen considerably once he becomes a legal adult.

Does your partner have full legal custody/sole responsibility for the child?

Is she currently resident in the UK? If so, how much does she earn?

You may want to look at moving to another EEA member-state before December this year and sponsoring him to that EEA member-state under EU law, which will allow him to join you and her before the age of 21.

Be aware that as your partner is a British citizen by descent herself and her son is almost an adult, he will not be able to claim citizenship via her and wil need to go through the whole naturalisation process just like any other foreigner.